airbrake manual english

AIR BRAKEY U K O N

AIR BRAKEM A N UA L

Y U K O N A I R B R A K E M A N U A L

FOREWORDIn keeping with the classified driver licensingsystem and with special regard to the air brakeendorsement, this manual has been compiled toassist you in your understanding of the basicfunctions of the air brake system.

The study of this manual, along with practicalinstruction, is recommended to persons who arep reparing for the completion of the air brakeexamination as administered by the Yu k o nDepartment of Community and TransportationS e rvices, Registry of Motor Vehicles, or anyapproved training institution.

Study questions are included so that you maytest your understanding of the subject.

This manual is presently used by a number ofjurisdictions in Canada and the United States.

Y U K O N A I R B R A K E M A N UA L

C O N T E N T S

SECTION I

BRAKES AND BRAKING

Heat, Energy, Traction, Friction ..................................... 6

Speed, Weight, Distance ................................................. 6

How We Obtain Force .................................................... 7

Stopping Distance ........................................................... 8

Section Summary Questions .......................................... 8

SECTION II

THE COMPONENTS

OF AN AIR BRAKE SYSTEM

Compressor ................................................................... 12

Reservoirs ...................................................................... 13

Foot Valve ..................................................................... 14

Brake Chambers and Slack Adjusters ........................... 14

Wedge Type Brake ........................................................ 16

Disc Brakes .................................................................... 16

Section Summary Questions ........................................ 17

SECTION III

HOW THE BASIC SYSTEM WORKS

Simple Air Brake System ............................................... 20

One-Way Check Valve .................................................. 20

Gauges ........................................................................... 21

Brake Application Gauge .............................................. 21

Low Pressure Warning .................................................. 21

Wig-Wags ...................................................................... 21

Stop Light Switch .......................................................... 21

Quick Release Valve ...................................................... 21

Relay Valve .................................................................... 22

Front Wheel Limiting Valve ......................................... 22

Tandem Rear Axles ........................................................ 23

Parking Brakes .............................................................. 24

Dual Control Valve and Reservoir System ................... 26

Mechanical Release ....................................................... 26

Air-actuated Hydraulic Brakes ...................................... 27

Section Summary Questions ......................................... 28

SECTION IV

TRAILER SYSTEMS

Glad Hands .................................................................... 30

Application Line ............................................................ 30

Hand Operated Valves ................................................... 31

Two-Way Check Valve ................................................... 32

Tractor Protection .......................................................... 33

Automatic Trailer Supply Valve System ........................ 33

Trailer Supply Valve ....................................................... 34

Tractor Protection Valve ................................................ 34

Tractor and Trailer Unit Coupled .................................. 36

Charging the Trailer System .......................................... 36

Brake Application – Foot Valve ...................................... 37

Brake Application – Hand Valve ................................... 37

Emergency Applications ................................................ 38

Service Line Rupture ...................................................... 38

Supply Line Rupture ...................................................... 39

Loss of Main Reservoir Air ............................................. 39

Manually Operated Trailer Supply Valves ..................... 40


SECTION V

DUAL AIR SYSTEMS

A Simple Dual Circuit Air System ................................. 44

Combination Truck and Trailer .................................... 46

T-75 Off-Highway System, Single Unit ......................... 47

L-75 Off-Highway System, Combinationwith Schedule X Trailer ............................................ 48

Schedule SX, Off-Highway Trailer withSpring Brakes ............................................................ 48

Section Summary Questions ......................................... 49

SECTION VI

PRE-TRIP INSPECTION

Single Unit ..................................................................... 52

Combination Unit ......................................................... 52

Brake Adjustment .......................................................... 53

Air Operated Disc Brakes ............................................... 53

In Service Check ............................................................ 53

Maintenance and Servicing of the Air Brakes .............. 54


Metric Conversion Table ................................................ 54

Y U K O N A I R B R A K E M A N U A L


BRAKES ANDBRAKING

HEAT, ENERGY,

TRACTION, FRICTION

SPEED, WEIGHT,

DISTANCE

HOW WE OBTAIN FORCE

STOPPING DISTANCE

SECTION SUMMARY

QUESTIONS

SECTION I

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To stop the vehicle in 1/10 of the time it took to acceleratewould re q u i re a stopping force of ten times the accelerationf o rce – the equivalent of approximately 2,000 horsepower.If the vehicle had six wheels, each wheel would have top rovide 1/6 of the braking force. If one or two of the wheelshad brakes that were not p roperly adjusted, the other wheelswould have to do more than their share of the braking, andthat might be more than their brakes were constructed tostand. Excessive use of the brakes would then result in abuildup of heat greater than the brake drums could absorband dissipate. Too much heat would result in brake damageand possible failure.

250˚ C 425˚ C 1100˚ CNORMAL MAXIMUM PANIC!

Most brake linings operate best at around 250˚ C andshould not exceed 425˚ C. It’s important to understandthat the power needed to stop generates heat which couldruin the brakes.

SPEED • WEIGHT •DISTANCEThe distance required to stop a vehicle depends on itsspeed and weight, in addition to the factors of energy,heat and friction. The brake force required to stop a vehi-cle varies directly with its weight and the “square” of itsspeed. For example, if the weight is doubled, the stoppingforce must be doubled to be able to stop in the same dis-tance. If the speed is doubled, the stopping force must bei n c reased four times to be able to stop in the same distance.And, when weight and speed are both doubled, the stop-ping force must be increased eight times to be able to stopin the same distance.

For example, a vehicle carrying a load of 14000 kgs forceat 16 km/h is brought to a stop in a distance of 30 metersby normal application of the brakes. If this same vehiclecarried 28000 kgs at 32 km/h, it would require eight timesthe braking force to stop the vehicle in 30 meters. Thiswould be more braking force than the brakes could pro-vide. No vehicle has enough braking force when it exceedsits limitations.

BRAKESAND BRAKING

HEAT • ENERGY •TRACTION • FRICTIONFor a vehicle to move along the highway, an internal combustion engine must convert its HEAT ENERGY intomechanical ENERGY. This mechanical ENERGY goes fromthe engine to the driving wheel tires by means of a systemof connecting rods, shafts and gears. The final factor thatmoves the vehicle is the amount of TRACTION its tireshave on the road surface. TRACTION is the ability of a tireto grip the road surface on which it rolls. The vehicle’sacceleration rate depends on the power the engine devel-ops and the amount of TRACTION the tires have on theroad surface.

FRICTION is the force which resists movement betweentwo surfaces in contact with each other. To stop a vehicle,the brake shoe linings are forced against the machinedsurfaces of the brake drums, creating FRICTION. ThisFRICTION produces HEAT.

The engine converts the ENERGY of HEAT into the ENER-GY of motion; the brakes must convert this ENERGY ofmotion back into the ENERGY of HEAT. The FRICTIONbetween brake drums and linings generates HEAT whilereducing the mechanical energy of the revolving brakedrums and wheels. The heat produced is absorbed by themetal brake drums, which dissipate the heat by passing itoff into the atmosphere. The amount of heat the brakedrums can absorb depends on the thickness of the metalof which they are made. When enough FRICTION is cre-ated between the brake linings and the drums, the wheelsstop turning; but the final factor that stops the vehicle isnot the brakes, but the TRACTION between the tires andthe road surface.

If an engine of 200 horsepower accelerates a vehicle to 100km/h in one minute, imagine the power needed to stopthis same vehicle. Not only that, but the vehicle mighthave to be stopped in an emergency, in as little as six sec-onds (just 1/10 of the time it took to reach 100 km/h).

S E C T I O N I Y U K O N A I R B R A K E M A N UA L

HOW WE OBTAIN FORCE

A. MECHANICALLY

Braking systems use devices to gain mechanical advantage.The most common device for this purpose is leverage.Look at this simple lever system:

A lever is placed on a pivot called the fulcrum. As the dis-tance from A to C is four feet, and from C to B is one foot,the ratio is four to one (4:1). Force has been multiplied bythe leverage principle. If a 100 lb. downward force is appliedat point A, then the upward force at point B is 400 lbs.

This is the result of the mechanical advantage of leverage.

C o m p a re the Points A, C, B to the previous lever d i a g r a m .

B. USE OF AIR

F o rce can also be multiplied by the use of air to gain a furt h e rmechanical advantage. Everyone has felt the force of air ona windy day. Air can be compressed (squeezed) into a muchsmaller space than that amount of air normally wouldoccupy. For instance, air is compressed in tires to supportthe weight of a vehicle. The smaller the space into whichair is squeezed, the greater the air’s resistance will be to beingsqueezed. This resistance creates pressure, which is used togain mechanical advantage.

If a constant supply of compressed air were directed thro u g ha pipe that was one inch square, and if a one inch squareplug were placed in the pipe, the compressed air would pushagainst the plug. Holding a scale against the plug wouldregister how many pounds of force were being exerted bythe air against the plug.

If the scale re g i s t e red ten pounds, for example, then it couldbe said the force was ten pounds on the one square inchsurface of the plug. This would be ten pounds per squareinch (10 P.S.I.) or 68.95 kPa (kilopascals).

The more the air in the supply tank has been compressed,the greater the force that would be exerted on the face ofthe plug.

C. LEVERAGE& AIR PRESSURE

In actual operation, pipes are round and plugs arediaphragms of flexible material acting against push rods.If compressed air of 120 P.S.I. acts on a diaphragm of 30square inches, 3,600 lbs., of force is produced (120 x 30).Apply this force to a push rod to move a 6-inch slackadjuster operating a cam and the total force equals 21,600inch pounds torque, (3,600 x 6) or 1,800 foot pounds torq u e(21,600 ÷ 12). It requires 25 - 30 foot pounds of torque totighten the wheel on a car. This comparison illustrates thef o rce obtained from using mechanical leverage and airpressure combined.

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STOPPING DISTANCEIn addition to the above factors, a driver must understandwhat is meant by the term “stopping distance”. Stoppingdistance consists of three factors:

Driver’s reaction time + Brake lag + Braking distance.

REACTION TIME: The time it takes from the moment ahazard is recognized to the time the brake is applied,approximately 3/4 of a second. (Reaction time is oftencalled “thinking time”.)

BRAKE LAG: The time the air takes to travel through aproperly maintained air brake system, (approximately 4/10

of a second).

BRAKING DISTANCE: The actual distance the vehicle travelsafter the brake is applied until the vehicle stops.

(The distance depends on the ability of the lining to pro-duce friction, the brake drums to dissipate heat and thetires to grip the road.)

The professional driver never takes the brakes for granted.The braking system must be tested and the adjustmentchecked before placing the vehicle into service. The pro-fessional understands the braking system, realizes thecapabilities and limitations, and learns to use them to thebest advantage.

Heavy vehicles require powerful braking systems that areobtained by use of mechanical leverage and air pressure.Brakes must be used keeping in mind the heat generatedby friction. If the heat becomes too great, braking effec-tiveness will be lost. The heavier the load and the fasterthe speed, the greater the power needed to stop.

Stopping distance is also affected by the driver’s reactiontime, brake lag, and braking distance. The pro f e s s i o n a ldriver is well aware that the vehicle, even with properlyadjusted brakes, will not stop as quickly as a passengerv e h i c l e .

C O M PA R ATIVE STOPPING DISTA N C E S

SECTION SUMMARY1. What is the final factor that will determine if the vehicle

will move?

2. What is the final factor that will determine if the vehiclewill stop?

3. How is the heat dissipated that is generated by thebrakes?

4. If one set of brake shoes are poorly adjusted, what eff e c tcould it have on the remaining sets of brake shoes inthe system?

5. What is meant by the term “FRICTION”?

6. If the weight of the vehicle is doubled, how many times must the stopping power be increased?

7. If both weight and speed of the vehicle a re doubled, how many times must the stopping power be increasedto be able to stop in the same distance?

8. If the speed of the vehicle is doubled, how many timesmust the stopping power be increased to be able to stopin the same distance?

9. What is compressed air?

10. What does the abbreviation “P.S.I.” stand for?

11. If 40 P.S.I. is exerted against a diaphragm of 30 squareinches in area, what is the total pounds of force thatcould abe exerted?

12. ”Brake lag” is approximately 4/10 of a second in a well-maintained, single circuit brake system. What is meant by “brake lag”?

13. What is meant by the following terms?“Reaction distance” – “Braking distance” – “Stoppingdistance”.

N O T E S

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S E C T I O N I Y U K O N A I R B R A K E M A N UA L

Section I of this manual has explained that it is possible to gaina mechanical advantage through the use of levers and that airunder pressure can be used to gain a mechanical advantage.Section II will explain how air under pressure can be used tooperate the air brakes of a vehicle.

Piping diagrams have been kept simple in order to be easilyunderstood. The piping arrangements found on vehicles inactual use on the highway might differ somewhat from theillustrations in this manual.

The diagrams in this manual ARE NOT to be interpretedas regulations for, or specifications of, air brake systems.

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Y U K O N A I R B R A K E M A N UA L S E C T I O N I

N O T E S N O T E S

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THE COMPONENTSOF AN AIR BRAKESYSTEM

COMPRESSOR

RESERVOIRS

FOOT VALVE

BRAKE CHAMBERS AND

SLACK ADJUSTERS

WEDGE TYPE BRAKES

DISC BRAKES

SECTION SUMMARY

QUESTIONS

11

SECTION II

Y U K O N A I R B R A K E M A N U A L S E C T I O N I I

THE COMPONENTS OF AN AIR BRAKE SYSTEMA basic air brake system capable of stopping a vehicle hasfive main components:

1. A COMPRESSOR, to pump air

2. A RESERVOIR OR TANK, to store the compressed air

3. A FOOT VALVE, to regulate the flow of compressed airfrom the reservoir when it is needed for braking

4. BRAKE CHAMBERS & SLACK ADJUSTERS, the means oftransferring the force exerted by the compressed air to mechanical linkages

5. BRAKE LININGS AND DRUM OR ROTORS, to create thefriction required to stop the wheels

It is necessary to understand how each of these componentsworks before studying their functions in the air brake s y s t e m .

THE COMPRESSORCompressed air is the means of transmitting force to an airbrake system. The source of the compressed air is the com-pressor. A compressor is designed to pump air. Air pumpedinto a tank will result in pressurizing the air, as wasexplained previously in this manual.

A piston type compressor operates on a similar principle tothat of the intake and compression strokes of an engine.

I N TAKE STROKE: The downward stroke of the piston cre a t e sa lower pressure within the cylinder than the atmosphericp re s s u re outside the compre s s o r. This causes air to flow pastthe inlet valve (which the atmospheric pre s s u re has opened)into the cylinder.

COMPRESSION STROKE: The upward travel of the pistonc o m p resses the air in the cylinder. The rising pre s s u re can-n o t escape past the inlet valve (which the compressed airhas closed) and as the piston nears the top of the stroke,the pressurized air is forced past the discharge valve andinto the discharge line leading to the reservoir.

UNLOADING: When sufficient pressure has been built up,the compressor goes into an “unloading” stage. The unloaderplunger holds the inlet valves off their seats until pressuredrops to the pre-set cut in pressure which is regulated bythe governor.

The compressor is driven by the vehicle’s engine, by eitherbelts and pulleys or shafts and gears. In vehicles which thecompressor is driven by belts, the belts should be checkedregularly for cracks and tension. Belt adjustment is usual-ly checked by pressing with a finger midway between thepulleys. A greater distance between the pulleys will affectthe amount of permissible slack. If the belts become tooslack, they could slip, and the compressor will not achieve

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S E C T I O N I I Y U K O N A I R B R A K E M A N UA L

its maximum eff i c i e n c y. While checking the drive belts alsocheck the compressor for broken mounting brackets orloose mounting bolts.

The compressor is in constant drive with the engine.Whenever the engine is running, so is the compressor. Itis usually lubricated from the engine lubrication system.Some compressors are self-lubricating and require regularchecks of lubricant levels.

There are periods of time when it is not necessary for thec o m p ressor to pump air. A common pre s s u re in an air brakesystem is from a low of 85 P.S.I. to a high of 105 P.S.I. Somesystems operate between 105 P.S.I. and 125 P.S.I. Minimump re s s u re is approximately 20 P.S.I. below maximum pre s s u re .

When the pre s s u re has reached the system’s maximum,the compressor goes into an “unloading” stage.

Most compressors have two cylinders. In the unloading stage,the inlet valves are held open, allowing the air to be pumpedback and forth between the two cylinders, instead of com-pressing it. During the “unloaded” stage, the compressoris able to cool.

It is most important that the air in the air brake system bekept as clean as possible. Dirt in the system can cause tro u b l e .

The air from the atmosphere that enters the compressormust first pass through a filter to remove any dust parti-cles from the air. The air filter must be cleaned regularly.A dirty filter will restrict the flow of air into the compres-sor, reducing its efficiency. Some vehicles have the inletport of the compressor connected to the air cleaner of thevehicle’s engine.

A governor controls the minimum and maximum air pres-sure in the system. As the compressor is in constant drivewith the engine, the governor’s job is to “unload” thecompressor when the desired pressure is reached. The gov-ernor does this by directing air pressure to the inlet valvesof the compressor, holding them open when pressure inthe system reaches its maximum. When the pressure inthe system drops by approximately 20 P.S.I., the governorallows the inlet valves to close, returning the compressorto its pumping stage.

THE RESERVOIRSR e s e rvoirs or tanks hold a supply of compressed air. A safetyvalve protects them from being over pressurized and burst-i n g. The safety valve protects the air brake system from anexcessive buildup of air pressure. The valve consists of aspring loaded ball which will allow air to exhaust thereservoir pressure into the atmosphere. The valve’s pres-sure setting is determined by the force of the spring. Asafety valve is normally set at 150 P.S.I. If the pressure inthe system rises to approximately 150 P.S.I., the pressurewould force the ball off its seat, allowing the pressure toexhaust through the exhaust port in the spring cage.When reservoir pressure is s u fficiently reduced (to appro x-imately 135 P.S.I.), the spring will force the ball back ontoits seat, sealing off the reservoir pressure. Not all safetyvalves have a manual release feature as shown in the dia-gram below.

If the safety valve has to relieve the pressure, it indicatesthat the governor or compressor requires service or repair.This should be done by a qualified mechanic.

IMPOR TANT! SAFETY VALVE ADJUSTMENTS MUST NOTBE TAMPERED WITH. ONLY QUALIFIED MECHANICSSHOULD SERVICE SAFETY VALVES.

Reservoirs or tanks are also equipped with drain cocks todrain off any moisture. When air is compressed, it becomesheated. The heated air cools in the reservoir, forming con-densation. Oil leaking past the piston rings of the com-pressor mixes with this moisture to form a sludge in thebottom of the reservoir. If allowed to accumulate, thissludge (water and oil) would enter the braking system. Anexcess of water in the system causes trouble with valvesand other parts. In winter, water in the system may freeze,causing malfunction of valves or brake chambers.

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To minimize the amount of water collection, all tanks mustbe drained daily. Under extreme conditions, tanks mayhave to be drained more than once a day. Park the vehicleon a level surface, set the parking brake, block the vehicle’swheels, open drain cocks fully. Allow all air pre s s u re toescape, which will then permit the moisture, collected inthe tank, to drain. Some tanks have more than one com-partment and each compartment has its own drain cock.Briefly opening the valve just to allow some of the air toescape DOES NOT drain the moisture!

Some re s e rvoirs are equipped with automatic re s e rvoir drainvalves (spitter valves). These may be drained manually bypushing up on a rod on the underside of the valve.

Most vehicles are equipped with more than one reservoir.This gives the system a larger volume of main reservoir air.As the first tank collects more of the moisture and oil fromthe compre s s o r, it is called the “wet tank”. The second tankis usually further from the compressor and so collects drieror cleaner air. This tank is called the “dry tank”.

Reservoirs vary in size. The size of the tanks to be installedon a vehicle depends on the number and size of the brakechambers. Brake chambers with a 30 square inch diaphragmwill obviously require a larger volume of reservoir air thanthose equipped with 20 square inch diaphragms.

THE FOOT VALVEThe foot-operated valve is the means of applying air tooperate the brakes. The amount of distance the treadle ofthe foot valve is depressed by the driver determines theamount of air pressure that will abe applied, but the MAX-IMUM APPLICATION WILL NOT EXCEED THE PRESSUREIN THE RESERVOIR. Releasing the foot valve treadle re l e a s e sthe brakes.

When the driver applies the brakes, depressing the treadlepart way, the foot valve will automatically maintain theapplication air pressure, without the driver having toadjust the foot pressure on the treadle.

Releasing the tread allows the application air to beexhausted through the exhaust ports to the atmosphere.Air treadles are spring loaded, producing a different “feel”from hydraulic brake application.

THE BRAKE CHAMBERS AND SLACK ADJUSTERS

A brake chamber is a circular container divided in the mid-dle by a flexible diaphragm. Air pressure pushing againstthe diaphragm causes it to move away from the pressure,forcing the push rod outward against the slack adjuster.The force exerted by this motion depends on air pressureand diaphragm size. If a leak occurs in the diaphragm, airis allowed to escape, reducing the effectiveness of thebrake chamber. If the diaphragm is completely ruptured,brakes become ineffective.

Front brake chambers are usually smaller than rear brakechambers because the front axles carry less weight thanthe rear axles. A brake chamber is usually mounted on theaxle, near the wheel that is to be equipped for braking. Airp re s s u re is fed through an inlet port. The air pushes againstthe diaphragm and the push rod. The push rod is connect-e d by a clevis and pin to a crank arm type lever called a“slack adjuster”. This converts the pushing motion of thepush ro d from the brake chamber to a twisting motion ofthe brake camshaft and “S” cams. When the air is exhausted,the spring in the brake chamber returns the diaphragmand push rod to the released position.

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As indicated by its name, the slack adjuster is also the meansof adjusting the “slack” or free play in the linkage betweenthe push rod and the brake shoes. This slack occurs as thebrake linings wear. If the slack adjusters are not adjustedwithin the limitations, effective braking is reduced andbrake lag time is increased. If too much slack develops, thediaphragm will eventually “bottom” in the brake cham-ber. If this condition is allowed to occur, the brake will notbe effective at all.

The diagram illustrates a common type of slack adjuster,showing the worm adjusting gear. When the brakes arefully applied, the angle between the push rod and the armof the slack adjuster should be no less than 90 degrees (aright angle). On this type of slack adjuster, the adjustingw o rm bolt tis turned until the brake linings touch the dru m sand then back off, normally 1/4 to 1/2 a turn. A lockingdevice, which may be a spring loaded collar over the headof the adjusting bolt, must be depressed when the wrenchis slipped over the bolt head. Some slack adjusters use aspring loaded internal check ball to lock the adjustment.The more often the driver checks the “slack” the less prob-ability of brake failure. Vehicles rarely “lose” their brakesbecause of loss of air; it is usually a loss of adjustment .

Some systems have automatic slack adjusters which adjustautomatically to compensate for brake lining wear, thusmaintaining the correct clearance between the brake liningand drum. Automatic slack adjusters must be checked reg-ularly to ensure that correct adjustment is being maintained.

NOTE: Detailed brake adjustment procedures are outlinedin Section VI.

The diagram below illustrates a common type of brakeassembly used on truck rear axles and trailer axles. A frontaxle assembly has the brake chamber and slack adjustermounted on the backing-plate because of the steering action.

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Brake lining material is attached to these shoes. The mate-rial used depends on the braking re q u i rements of the vehicle.Brake lining must give uniform output of brake effort withminimum fade at high temperature.

Fading or a reduction in braking effort occurs when theheated drums expand away from the brake linings. Thebrake linings also lose their effectiveness with overh e a t i n g .

The twisting action of the brake cam shaft and “S” camf o rces the brake shoes and linings against the drums. Thebrake linings generate heat from friction with the braked rum surf a c e

The thickness of the drums determines the amount of heat they are able to absorb and dissipate to the atmosphere .D rums worn thin will build up heat too quickly.Dangerously undependable brake performance will resultf rom distorted drums, weak re t u rn springs, improper lining,poor adjustment, or grease or dirt on the lining.

Drums must never be turned or worn beyond the manu-facturer’s specifications.

WEDGE TYPE BRAKESHere is another example of a type of brake assembly usedon some air brake equipped vehicles. The action of thebrake chamber push rod forces a wedge shaped push rodbetween the brake shoe rollers. This forces the brake shoelining against the brake drum.

The vehicle may be equipped with a single chamber ortwo chambers on each wheel, depending on the vehicle’ssize and style.

These brakes may be equipped with a self-adjusting mech-anism or with a manual “star wheel” adjuster. The “starwheel” adjustment is made with the vehicle jacked up, toensure that the brake linings do not drag. Manual adjust-ment of wedge type brakes is usually a job for a mechanic.

Spring brakes added to these units are of the piggy-backtype.

DISC BRAKES

The air activated heavy truck disc brake is similar in principle to the disc brake used on passenger vehicles. Airpressure acts on a brake chamber and slack adjuster, acti-vating the brakes. For light truck and passenger car brak-ing systems, hydraulic pre s s u re is used. Instead of the camor wedge used in conventional heavy truck drum brakes,a power screw is used. A “power screw” works like a C-clamp,so that the lining pads exert equal force to both sides ofthe disc or ro t o r. Some types if disc brakes have a built-inautomatic adjuster. Disc brakes that re q u i re manualadjustment have adjustment specifications different fromconventional “S” cam braking systems. ALWAYS CHECKM A N U FACTURER’S SPECIFICATIONS BEFORE ADJUSTING.

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SECTION SUMMARY1. What are the five basic components of an air brake

system?

2. What are two standard main re s e rvoir pre s s u res for theoperation of a vehicle’s air brakes?

3. At what pressure should the governor cause the com-pressor to return to its pumping stage?

4. How is a plugged air filter likely to affect the air com-pressor?

5. What causes moisture to form in the air brake system?

6. When is the compressor able to accomplish most of itscooling?

7. How are most compressors lubricated?

8. How often should the reservoirs be drained?

9. Is it necessary to allow all the pressure to escape fromthe re s e rvoir in order to remove the moisture and sludgewhich may have accumulated?

10. What is the maximum pre s s u re available for a full brakeapplication at any given time?

11. What will result if the brake drums are worn thin orturned too far?

12. If the governor valve failed to “unload” the compre s s o r,what would be the result?

13. What is the purpose of having more than one re s e rv o i r.

14. What are two functions of the slack adjuster?

15. Does the amount of slack in the brake linkages haveany effect on the braking efficiency of the vehicle?

16. What is the advantage of keeping the brake chamber push rod travel adjusted to minimum travel?

17. What is the most common cause of loss of effective braking in an air brake system, loss of air pressure or loss of brake adjustment?

18. Do automatic slack adjusters on “S” cam brakes re q u i rechecking?

19. Can the adjustment on air operated disc brakes differfrom “S” cam type brakes.

20. What occurs when drum brakes become overheated?

21. What causes brake fade at high temperatures?

22. What is the main function of the foot valve?

23. Why does the “feel” of an operated foot valve differfrom a hydraulic brake pedal?

24. On what principle does a disc brake operate?

N O T E S

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Y U K O N A I R B R A K E M A N UA L S E C T I O N I I

N O T E S N O T E S

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HOW THE BASICSYSTEM WORKS

SIMPLE AIR BRAKE SYSTEM

ONE WAY CHECK VALVE

GAUGES

BRAKE APPLICATION GAUGE

LOW PRESSURE WARNING

WIG-WAGS

STOP LIGHT SWITCH

QUICK RELEASE VALVE

RELAY VALVE

FRONT WHEEL

LIMITING VALVE

TANDEM REAR AXLES

PARKING BRAKES

DUAL CONTROL VALVE

AND RESERVOIR SYSTEM

MECHANICAL RELEASE

AIR-ACTUATED

HYDRAULIC BRAKES

SECTION SUMMARY QUESTIONS

SECTION III

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HOW THE BASICSYSTEM WORKS

A SIMPLE AIR BRAKE SYSTEMN O T E : ALL PIPING DIAGRAMS ARE USED TO ILLUS-TRATE BASIC BRAKING PRINCIPLES ONLY AND DO NOTILLUSTRATE ACTUAL AIR BRAKE SYSTEMS.

BASIC OPERATION PRINCIPLE

Air is pumped by the compressor (1) to the reservoir (2),which is protected from over pressurization by a safetyvalve (3). The governor (5) controls the pre s s u re in there s e rvoir by governing the compre s s o r. Air is available fro mthe re s e rvoir to the bottom of the foot valve (6). The driverpushes down the foot valve treadle and air pre s s u re flows tothe front and rear brake chambers (7,8). The brake chamberpush rods move the slack adjusters. The slack adjusters ro t a t ethe “S” cams, forcing the brake shoe against the drums.This causes friction which stops the wheels. The driverreleases the foot valve treadle, and the air in the brakechambers is allowed to exhaust through the foot valve,releasing the brakes.

Other valves are necessary to ensure smooth and efficientoperation.

ONE-WAY CHECK VALVEIn the diagram below, two reservoirs are shown. This givesthe system a larger volume of main reservoir air. The first

first tank in the system collects most of the moisture, andis referred to as the wet tank. The second tank is referredto as the dry tank. Reservoirs vary in size. The size of thetank or tanks installed on a vehicle depends on the num-ber and size of brake chambers.

To prevent air from flowing back from the reservoirs to thec o m p re s s o r, a ONE-WAY CHECK VA LVE (9) is installed. Thisvalve allows the air to flow in one direction only. Thevalve is spring loaded. Pressure at the inlet side overcomesthe spring pressure and lifts the check valve ball, or disc,off its seat. Air passes through the valve to the outlet. Whenp re s s u re at the outlet becomes greater than at the inlet,t o g e t h e r with the spring pre s s u re, the check device seats,p re v e n t i n g air from flowing back through the valve.

20

S E C T I O N I I I Y U K O N A I R B R A K E M A N UA L

GAUGESAll vehicles are equipped with a pressure gauge (10) toindicate the amount of air pressure in the main reservoirsystem. This gauge is mounted in the cab, usually on thed a s h b o a rd. Common operating pre s s u res are 85 P.S.I. to105 P.S.I. and 105 P.S.I. to 125 P.S.I., depending on the system.M o n i t o r i n g the gauge will alert the driver to any unusualchanges in pressure.

BRAKE APPLICATION GAUGEAn additional gauge can be installed on the dash to indi-cate the application pressure when the brakes are applied.This gauge can be piped to indicate the pressure of eithera foot application or of a hand application. (Hand appli-cation will be explained later in the manual.)

LOW PRESSURE WARNINGAll vehicles equipped with air brakes must have a deviceto warn the driver if the air pressure in the system dropsto a dangerous level.

Should the air pressure drop below approximately 60 P.S.I.due to overuse or leaks, the low pressure indicator switch(11) will turn on a red warning light on the dash, or causea buzzer to sound. Some vehicles are equipped with both al i g h t and a buzzer to warn the driver of a low air pressurecondition.

WIG-WAGSThere are two common types of “wig-wag” low pressurew a rning devices that may be used. Both types will drop intothe driver’s view when the pressure in the system dropsbelow approximately 60 P.S.I. The automatic warningdevice will rise out of the driver’s view when the pressurein the system rises above 60 P.S.I. The manual reset typemust be placed in the “out of view” position manuallyand will not stay in place until the pressure in the systemgoes above 60 P.S.I.

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Whichever warning system is used, buzzer, lights or wig-w a g , the driver MUST STOP the vehicle and find the causeof the air loss. The air pressure remaining in the system(approximately 60 P.S.I.) is enough for brake application ifthe driver acts promptly.

STOP LIGHT SWITCHAny driver following must be warned that speed is beingreduced or the vehicle is being stopped. The STOP LIGHTSWITCH (12) is an air operated electric switch which turn son the brake lights on the rear of the vehicle when a brakeapplication is being made.

QUICK RELEASE VALVEThe application of the brakes in the simple system wasdescribed earlier. In such a basic system, when the driverreleases the foot valve, it would be necessary for the air


under pressure in the brake chambers to return back to thefoot valve to release the brakes. This releasing action wouldbe slowed in long wheel base vehicles, due to longer linesbetween the foot valve and the rear brake chambers. Toallow the brakes to release quickly and fully, by discharg i n gthe application air near the brake chambers, a quick re l e a s evalve (13) may be installed.

RELAY VALVEThe foot valve is usually located closer to the front wheelsthan to the rear wheels. The longer the distance from thefoot valve to the rear chambers, the more time it will take(known as brake lag) before the rear brakes apply. To cor-rect this condition on a long wheel base vehicle, a RELAYVA LVE (14) is installed near the rear brake chambers. Alarger diameter pipe is connected between the main reser-voir and the relay valve. The air line from the foot valve tothe relay valve now becomes a “control line”. (The air inthe control line “dead ends” at the relay valve.) When thefoot valve is depressed, the air pressure in the control lineacts on the top section of the relay valve, causing the re l a yv a l v e to “relay” reservoir air directly to the rear brakechambers through the larger diameter pipe. The pressureof the reservoir air delivered in this way will be the sameas the control pre s s u re delivered by the foot valve.Releasing the foot valve exhausts the control air to therelay valve, allowing it to cut off the flow of reservoir airto the rear chambers, in turn exhausting the air in thebrake chambers by the quick release feature of the relayvalve.

The driver places the control valve in the “normal” positionfor dry road surfaces and the front braking application pre s-s u re is normal. On a slippery road surface, the driver mayflip the control valve to the “slippery road” position. Inthis position, the control valve will cause the LIMITINGVALVE to operate. Application air pressure to the frontwheels is then reduced to 50 percent of the application airpressure being delivered to the rear brake chambers.

This valve will hold off brake application to the front wheelsfrom 0 to 10 P.S.I., depending on how it has been preset.Between the preset pressure and 40 P.S.I. of brake applica-tion, the reduction is approximately 50 percent. Brakeapplications between 40 P.S.I. and 60 P.S.I. are reduced byless than 50 percent. Brake applications over 60 P.S.I. arenot reduced; full application pressure is directed to thefront wheels.

FRONT WHEEL LIMITING VALVEFor better steering control on a slippery road surface, itcan be an advantage to reduce the braking effect to thef ront wheels. This can be accomplished by installing acontrol valve (15) in the cab, and a FRONT WHEEL LIMI-TING VALVE (16) on the front axle.

Some systems are equipped with an AUTOMATIC LIMITINGVALVE. (17)

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CHECK YOUR UNDERSTANDING

From the above diagram name each numbered compo-nent and briefly describe its function in the system. Writeyour answers on a separate sheet of paper.

TANDEM REAR AXLESThe air brake system discussed previously is that of a vehicleequipped with a single rear axle. The diagram illustrates

an air brake system of a vehicle equipped with an auto-matic front wheel limiting valve (17), a quick release valve(13) and a tandem set of rear axles. Both axles of the tan-dem set are equipped with brakes.

A relay valve (14) is used to provide quicker applicationpressure to the tandem rear axles.

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PARKING BRAKES(SPRING BRAKES)

Spring type parking brakes may be installed on an air brakeequipped vehicle to ensure a reliable parking brake system.In the service brake system, the brakes are held retractedby springs, and applied by air pressure. Spring type park-ing brakes are applied and remain applied WITHOUT AIRPRESSURE. The parking brake chambers are attached to theservice brake chambers and operate the brakes throughthe same linkage. Therefore, the effectiveness of the park-ing brake depends on the service brake adjustment. A con-t rol valve (usually a square, yellow button) in the cab allowsthe driver to exhaust air out of the parking brake circuit toapply the brakes, or to repressure the circuit to releasethem. The system can also act as an emergency brake. Lossof air from the main system may automatically apply thebrakes, depending on how the system is piped.

During normal driving, air pressure cages the spring, hold-ing it ready for parking or emergency braking.

Application of the dash control valve exhausts air fromthe spring brake chamber, causing spring force to activatethe spring brake.

During normal service brake operation, the spring brakedoes not apply. Air pressure keeps the spring caged.

PARKING BRAKE SYSTEMS

The installation of parking brakes and their piping arr a n g e-ments into a vehicle air brake system will vary dependingon the vehicle make.

Control valves will vary, depending on the manufacturerand type of piping arrangements.

This type of spring loaded valve requires that the driverpush the button to release the parking brakes. This valvecannot be left in the released position below appro x i m a t e l y35 P.S.I. pre s s u re in the main re s e rvoir system. Any time themain reservoir pressure drops to approximately 35 P.S.I.,this valve will exhaust automatically, placing the parkingbrakes into full application. Similar types of spring loadedvalves require that the driver pull the button out to releasethe parking brakes.

There is a single type of push-pull control valve in use thatdoes not have an automatic release feature. To apply theparking brakes, the valve must be operated manually, eventhough the main reservoir pressure has been emptied.

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USING THE PARKING BRAKES

The above diagram illustrates spring parking brakes (18)added to the brake chambers of the rear axle on the singleunit vehicle. A control valve (19) is mounted on the cab.A supply line of reservoir air is piped from the dry tank tothe control valve. Opening the control valve admits mainreservoir air pressure to the parking brake units, releasingthem.

Closing the control valve shuts off the supply of reservoirair pressure and exhausts the existing pressure in the park-ing brake units, thus applying the brakes.

SINGLE CONTROL PARKING BRAKE SYSTEM

The diagram below illustrates the use of a third tank witha single control, used on the rear axle of a single unit.

A control valve (19) is mounted in the cab. A supply lineof reservoir air is piped from the dry tank through a one-way check valve (9) into a third tank. Closing the controlvalve shuts off the supply of re s e rvoir air pre s s u re andexhausts the existing pressure in the parking brake units,allowing them to apply the brakes. If the main reservoirair in the system should be emptied, the parking brakeswill not apply, because the one-way check valve will iso-late the pressure in the third tank. If the spring (parking)brakes are to be used to stop the vehicle, the driver willhave to set the parking brakes manually with the cab c o n t rol valve.

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DUAL CONTROL VALVEAND RESERVOIR SYSTEMSome vehicles, such as buses, may also be equipped withan EMERGENCY RELEASE TANK (2).

In this system, if main reservoir pressure is lost, the springbrakes will automatically apply.

If the parking brakes have applied because of a loss ofmain reservoir air, the driver can draw reserve air from thee m e rgency release tank to release the parking brakes. Adual parking brake control (19) is used together with thee m e rgency release tank. The driver must press on thee m e rgency release button. Releasing the emerg e n c yrelease button will automatically allow the parking brakesto re-apply.

The emergency release would only be used to move thevehicle from an undesirable location, if the parking brakeshad been applied due to a low main reservoir air condi-tion.

CAUTION: Parking brakes should be in the released posi-tion before making a service brake application. A full brakeapplication, made when the parking brakes are applied, cancompound the force exerted on the slack adjusters andlinkage which could result in damage or brake failure .Compounding is the combination of two forces; the forceapplied by the spring brakes and the force applied by theservice brake.

NOTE: Spring type brakes are primarily a parking brake,but in the event of loss of main reservoir air, they canassist in stopping the vehicle. How quickly they will stopthe vehicle depends on such factors as:

• The weight and speed of the vehicle;• The steepness of the grade;• The spring force of the spring brakes which have been

installed; and,• The adjustment of the service brakes.

MECHANICAL RELEASESome spring type parking brakes can be released mechan-ically by “winding them off” or “caging” them. A bolt,which runs through the centre of the chamber body, isturned to compress the spring. It may be necessary to firstremove a lock plate and stud, to gain access to the head ofthe bolt.

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Other types have a plug which must first be removed anda bolt inserted. In some cases, a special wrench is required.Instruction on how to “cage” is usually on the body of theparking brake. If all air is lost and the vehicle has to betowed, the parking brake can be released by winding themoff. Always block the wheels when caging the parkingbrake spring. Caging means the brakes are being released.

WARNING!

Parking brakes should never be disassembled withoutfirst compressing the spring with a wind-off bolt. Thesesprings are under extreme pressure and could cause seri-ous personal injury if disassembly is attempted by any-one not experienced in servicing these units. Disassemblyis a job for a mechanic.

AIR-ACTUATED HYDRAULIC BRAKES

An air-actuated braking system consists of a compressor,governor, air storage tanks, dual circuit foot valve and twobrake chambers. The system may also include spring typeparking brakes. The brake chambers are attached to twomaster cylinders that direct hydraulic pre s s u re to thefoundation brakes.

When the foot valve is depressed, air pr e s s u r e flows i n t othe primary (rear brake) and secondary (front brake) circ u i t s .The air pre s s u re in each circuit acts upon that circ u i t ’sbrake chamber which transmits force to the master cylinder.Each master cylinder in turn transmits hydraulic pressureto the wheel cylinders, actuating the front and rear axleservice brakes.

When the driver pushes the parking brake control valve tothe released position, air pre s s u re behind a diaphragm in thespring brake chamber compresses the spring and main-tains the parking brake in a released position. When theparking brake is applied by pulling the parking brake handvalve out, air in the diaphragm chamber is released andthe spring extends, applying the rear brakes.

If the main re s e rvoirs drop to approximately 35 psi, theparking brake valve will exhaust automatically, placing theparking brakes into full emergency/parking application.

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SECTION SUMMARY1. How can the driver tell how much air pressure there

is in the main reservoirs?

2. What must the driver do when a low pre s s u re warn i n gsystem activates?

3. What is the purpose of a quick release valve?

4. What is the purpose of a relay valve?

5. What is the purpose of using a larger diameter pipe between the main reservoir tank and the relay valve?

6. If the front axle limiting valve is in the “slippery ro a d ”position, and the foot valve is depressed to make a brake application of 30 P.S.I., how much pressure willbe applied to the front brake chambers?

7. How is the re s e rvoir protected from over pre s s u r i z a t i o n ?

8. What stops pressurized air from flowing back into thecompressor from the dry tank?

9. At what pre s s u re should the low pre s s u re warn i n gdevice activate?

10. How is “brake lag” to the rear wheels minimized?

11. When should a driver use the front wheel limiting valve?

12. What is meant by “compounding” the brakes?

13. Why are spring brakes a reliable type of parking brake?

14. How are parking brakes held in the released position?

15. What are the functions of the cab-mounted parkingbrake control valve?

16. Will parking brakes apply “automatically” in all brakingsystems?

17. What is the reason for releasing the parking brakes before making a full brake application test?

18. What is the purpose of an emergency release reservoirtank in a parking brake system?

19. How can some types of parking brakes be releasedwithout the use of air pressure?

20. What is the danger of disassembling a parking brakeunit?

N O T E S

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TRAILER SYSTEMS

GLAD HANDS

APPLICATION LINE

HAND OPERATED VALVE

TWO-WAY CHECK VALVE

TRACTOR PROTECTION

AUTOMATIC TRAILER SUPPLYVALVE SYSTEM

TRAILER SUPPLY VALVE

TRACTOR PROTECTION VALVE

TRACTOR ANDTRAILER UNIT COUPLED

CHARGING THE TRAILER SYSTEM

BRAKE APPLICATION – FOOT VALVE

BRAKE APPLICATION – HAND VALVE

EMERGENCY APPLICATIONS

SERVICE LINE RUPTURE

SUPPLY LINE RUPTURE

LOSS OF MAIN RESERVOIR AIR

MANUALLY OPERATEDTRAILER SUPPLY VALVES

SECTION SUMMARY QUESTIONS29

SECTION IV

Y U K O N A I R B R A K E M A N U A L S E C T I O N I V

TRAILER SYSTEMSIt can be assumed that the system illustrated up to thispoint is the air brake system of a truck unit. If a trailer wascoupled to this unit, the brakes of the trailer would haveto be operated from the tractor.

The following diagram shows the piping of a unit, similarto the tandem axles of the truck unit. The trailer has a sin-gle axle equipped with brake chambers.

A “TEE” has been installed in the application line betweenthe foot valve and the truck axle system. A line has beenconnected from this tee to the trailer by a set of couplerscalled “GLAD HANDS” (20).

GLAD HANDSThis term refers to the coupling device used to connect thes e rvice and supply lines of the trailer to the truck or tractor.

These couplers have a snap-lock position, and a ru b b e rseal prevents air from escaping.

Before connection is made, couplers should be clean andf ree of dirt and grit. When connecting the glad hands, startwith the two seals together and the couplers at a 90 degreeangle to each other. A quick snap downwards will join andlock the couplers. Vehicles equipped with “dead-end” cou-plers should have these protection plates in use wheneverthe vehicle is used without a trailer. This will prevent waterand dirt from entering the coupler and lines.

If the unit is not equipped with dead-end couplers, theglad hands of the service line can be locked to the gladhand of the supply line to keep water and dirt from enter-ing the unused lines. The cleaner the air supply is kept,the less chance of brake problems!

Glad hands and lines should also be secured to preventthe line from bouncing off the vehicle. This could seriouslydamage the couplers.

THE APPLICATION LINEThis application line is referred to as a SERVICE LINE. Inthis simple system used for an example, the driver depre s s e sthe foot valve treadle, and application air will be deliveredto the tractor brake chambers and to the trailer brakechambers. When the driver releases the foot valve treadle,the application air to the trailer brake chambers must re t u rnto the foot valve to be exhausted to the atmosphere.

In this example system, there would be disadvantages:

If the trailer broke away from the tractor, the trailer wouldhave no brakes.

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S E C T I O N I V Y U K O N A I R B R A K E M A N UA L

If the service line parted or ruptured, the trailer brakescould not be applied, and the application air would be lostfrom the tractor if a brake application were made.

If the air in the main reservoirs were lost, there would beno way to apply the brakes of the tractor or the trailer.

The trailer brakes cannot be applied independently fromthe tractor, and there is no way to set the trailer brakeswhen coupling to the tractor.

The application and release of the trailer brakes would beslower than those of the tractor.

These disadvantages are overcome by the addition of thelines and valves discussed in the following pages.

HAND VALVE AND TWO-WAYCHECK VALVETo provide a brake application to the trailer unit only, aHAND VALVE (21) and a TWO-WAY CHECK VALVE (22)are piped into the system.

The purpose of the hand valve is to allow the driver contro lindependently the amount of application air to be dire c t e dto the trailer brakes when a trailer brake application is beingmade. It also provides a method of applying the trailerbrakes when coupling the trailer to the tractor.

The purpose of a two-way check valve is to allow controlof the trailer brake by use of the hand or foot valve. Thisvalve will permit air to flow from the source which is sup-plying the higher application pressures.

HAND OPERATED VALVES

For trucks that are intended to pull trailers, the hand oper-ated valve is added to the air brake system to operate thetrailer brakes. This valve allows the driver to apply thetrailer brakes independently of the tractor. The amount ofapplication air pressure delivered depends on the amountthe valve is opened by the driver. Some valves must beclosed by the driver; others will close automatically fro many open position. Valves may be of a type that will remainin the full open position only, or they may be equipped witha locking device that will hold them in the desired position.

NOTE: The hand valve is NOT to be used for parking, asair may bleed off if the engine is stopped or the hand valvemoves to the released position.

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A TWO-WAY CHECK VALVE

This valve allows air to be directed to one delivery pipefrom either of two sources. A two-way check valve allowsthe source applying the higher pre s s u re to shift the shuttleso that the higher pressure will be directed to the deliveryor “service line.” This valve is used between the foot oper-ated valve and the hand operated valve, for the purpose ofindependently controlling the trailer brakes.

The driver has applied the brakes using the foot valve.Application air is directed to the brake chambers of thetractor AND to the trailer brakes through a two-way checkvalve (22). The shuttle has shifted to the low pressure side,closing off any air flow toward the hand valve side. Thehand valve (21) is in the closed position and equal pres-sure is being applied to the brake chambers of the tractorand the brake chambers of the trailer.

In this diagram, with the foot valve released, and the handvalve (21) opened, application air is directed from thehand valve through the two-way check valve (22), to the

brake chambers. The two-way check valve in this applica-tion has shifted to the low pressure side, closing off any airflow toward the foot valve side.

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The amount of application pre s s u re through the hand valvedepends on the amount that the valve is opened by thedriver. (But it cannot exceed main reservoir pressure!)

Any time a trailer brake application is made by use of thehand valve, the driver may depress the foot valve treadle.If the foot valve application is of a higher pressure thanthat of the hand valve, the two-way check valve will shiftto the lower pressure side, allowing the higher pressure tobe directed to the tractor and trailer brakes.

If, during a foot valve application, the driver makes ahigher pre s s u re hand valve application, the two-way checkvalve will allow the higher hand valve application to bedirected to the trailer brakes.

Regardless of whether the trailer brakes are applied inde-pendently by means of the hand valve, or together with thetractor brakes by the use of the foot valve, the maximumapplication pre s s u re can only be the same as, or slightly lessthan, MAIN RESERVOIR PRESSURE.

TRACTOR PROTECTIONA tractor protection system is added to prevent total lossof air from the tractor if the trailer breaks away, or if theconnecting air lines between tractor and trailer becomeseparated or ruptured. The tractor protection system con-sists of two valves added to the tractor system: the TRAC-TOR PROTECTION VALVE (23), and the TRAILER SUPPLYVALVE (24). Other names for the trailer supply valve are“trailer parking control” and “emergency valve.”

There are two types of trailer supply valves in use. Themost common is a spring loaded valve that , after beingopened manually, is held open by air system pre s s u re. Thisdevice is called an automatic trailer supply valve.

Some tractors have a manual trailer supply valve. Thesemay be a toggle type switch or push/pull type.

The tractor protection valves vary in outward appearance,depending on make and model, but all perform the samefunction: protecting the tractor air supply.

AUTOMATIC TRAILERSUPPLY VALVE SYSTEMThe diagram below illustrates that the main reservoir air ispiped from the main re s e rvoir lines to the trailer supplyvalve (24). The tractor protection valve (23) is fed by twolines, one from the trailer supply valve (24) and one fromthe two-way check valve (22). Leading off from the tractorprotection valve are two lines, each with a glad hand cou-pler. These two lines are referred to as the SERVICE LINEand SUPPLY LINE.

In the diagram, the upper line is the service line and thelower line is the supply line.

To understand the function of the trailer supply valve andthe tractor protection valve in the system, it is importantto understand how they operate.

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TRAILER SUPPLY VALVEThis valve (usually a red octagonal button) is mounted inthe cab of the vehicle, easily accessible to the driver. Thedriver opens the valve by pushing or pulling the button,depending on the type used.

Opening the valve permits main reservoir pressure to flowthrough the valve. This pressure is piped to the tractor pro-tection valve and the supply line glad hand. The valve isspring loaded, and will be held in the open position whens u fficient pre s s u re is reached. If the pre s s u re drops to a rangeof between 45 - 20 P.S.I., the valve will shut automatically,by spring pre s s u re, opening the exhaust port. The driver canclose the valve manually, to uncover the exhaust port.

TRACTOR PROTECTION VALVEThis is an example of a tractor protection valve, which isusually mounted on the cab or chassis of the tractor.

The diagram below illustrates a tractor unit equipped witha trailer supply valve (24) and a tractor protection valve (23).The trailer is not coupled and the tractor is being operatedas a single unit. The driver has not opened the trailer supplyvalve (24) and the hand valve (21) is closed.

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In the diagram above, the driver has made a brake appli-cation with the foot valve (6), and application air is beingd e l i v e red to the tractor brake chambers. The two-way checkvalve (22) has shifted to the low pressure side, allowingapplication air in the control line to reach the tractor pro-tection valve (23).

There is no air loss from the tractor through the discon-nected glad hands (20).

If the driver, by mistake, applied the hand valve (21), withthe trailer disconnected, the application air directed to thetractor protection valve would also be dead-ended. Again,no air loss would occur, if the trailer supply valve (24) is int h e closed position.

35


TRACTOR AND TRAILER UNIT COUPLEDIn the diagram above, the trailer has been coupled to thetractor, and the service and supply lines of the units havebeen coupled by using glad hands.

The trailer unit has a reservoir (2) installed. This tank willprovide a volume of air near the trailer chambers for nor-mal or emergency braking. The tank is equipped with adraincock.

A RELAY EMERGENCY VALVE (25) is mounted on thetrailer reservoir. (This valve can also be mounted directlyon the trailer frame near the brake chambers.) The relaye m e rgency valve serves three main functions in the system:

1. The RELAY part of the valve relays air from the trailerreservoir to the trailer brake chambers during a brakeapplication. This part of the valve operates like the re l a yvalve previously discussed. It also provides a quick re l e a s eof the trailer brakes.

2. The EMERGENCY part of the valve directs trailer reser-voir pre s s u re to the trailer brakes, causing an emerg e n c yapplication (“dynamiting”). This action will occur auto-matically in the event of ruptured or parted air lines between tractor and trailer, or loss of air from the main

re s e rvoir system. The driver may operate the cab-mounted trailer supply valve (24) to cause an emerg e n c ya p p l i c a t i o n of the trailer brakes.

3. The relay emergency valve has a one-way check valvewhich stops air in the reservoir from going back to the source of the supply.

CHARGING

THE TRAILER SYSTEMIn the previous diagram, the compressor has raised the mainreservoir pressure to maximum.

In the next diagram, the driver has opened the trailer supplyvalve (24) to allow the main reservoir air pressure to bedirected through the tractor protection valve (23) to thet r a i l e r. The air pre s s u re passes through the relay emerg e n c yvalve (25) to the reservoir on the trailer. Pressure will buildup in the trailer reservoir to the same pressure as the mainreservoirs on the tractor. This is known as “charging” thetrailer system. The trailer supply valve will remain in theopen position when the pressure has built up to between20 and 45 P.S.I., depending on the make.

36


BRAKE APPLICATION –FOOT VALVEThe diagram above illustrates the air flow action during abrake application being made with the foot valve (6). Theapplication air has applied the tractor brakes and the trailerbrakes together. As previously explained, the two-waycheck valve (22) has shifted, and application air is beingd i rected through the tractor protection valve (23) to theservice line.

This control pressure moves through the service line to acton the relay emergency valve (25). This control pressurewill cause the relay emergency valve to direct reservoir airfrom the trailer tank to the trailer brake chambers (26).This will be of the same P.S.I. as the control pre s s u re, whichwas the P.S.I. of the application air by the foot valve. Insuch a system, brake lag time has been minimized.

Release of the foot valve treadle stops the flow of applica-tion air. The relay portions of the valves return to theiroriginal positions, stopping the flow of air pressure. Theexhausting ports of the valves exhaust the air pressurefrom the brake chambers, releasing the brakes.

In this system, the brakes of both units can be releasedquickly.

BRAKE APPLICATION –HAND VALVEThe driver can use the hand valve (21) to apply the trailerbrakes. The air flow is illustrated in the next diagram. Thetractor protection valve (23) and relay emergency valve(25) are operated by the application air, as was explainedin the foot valve application.

Closing the hand valve will release the brakes by closingoff application air. The air pressure in the chambers andlines will exhaust, also as explained in the previous footvalve application.

CAUTION: TRAILER BRAKES MUST NOT BE USED TOHOLD A PARKED VEHICLE THAT IS LEFT UNATTENDED.LOSS OF PRESSURE WILL RESULT IN LOSS OF BRAKES!ALWAYS SET THE PARKING BRAKE.

37


EMERGENCYAPPLICATIONSA TRAILER BREAKAWAY would result in a separation ofthe service line and the supply line. The sudden loss of airpressure in the supply line will trigger the relay emergencyvalve (25) which causes the trailer reservoir (2) to dump itsair directly to the trailer brake chambers (26). This placesthe trailer brakes into emergency application.

The loss of pre s s u re in the supply line also causes the trailersupply valve (24) to automatically shift to the closed position.

The tractor brakes are operable, without air loss, because thetractor protection system has isolated the tractor.

The trailer brakes will remain applied until either the pres-s u re in the trailer re s e rvoir and lines is drained off, or thesupply line is repaired and the system is recharged.

SERVICE LINE RUPTUREIf the service line is ru p t u red or disconnected, no action willtake place until a brake application is made.

In this diagram, the service line has ru p t u red, and the driverhas made a brake application with the foot valve (6).

The application air is directed to the control line throughthe tractor protection valve (23). Rupture of the service linewill result in the escape of air pressure, causing a loss ofp re s s u re in the tractor system. This pre s s u re drop willcause the tractor protection system to close off, exhaust-ing the supply line to the trailer. This will cause the trailerbrakes to go into an emergency application.

The trailer brakes may be released by opening the trailersupply valve (24) to recharge the trailer system. A furtherapplication of the foot valve or hand valve would againtrigger an emergency application.

N O T E : Depending on the type of tractor protection systemused, the air loss from the tractor will stop immediately, orit will blow down to between 45 and 20 P.SI. and thenshut off .

38


SUPPLY LINE RUPTUREA rupture of the supply line (or an uncoupling of the sup-ply line glad hands) will result in a drop of pressure in thesupply line between the trailer supply valve (24) and therelay emergency valve (25). This will trigger the emerg e n cyaction of the relay emergency valve, placing the brakesinto emergency application. As in the previous examples,the trailer supply valve (24) will shift to the closed position.

The operation of the tractor brakes will not be affected, ifthe tractor protection system is in working condition.

The relay emergency valve (25) must be the no bleed-backtype, so no air is lost from the trailer.

LOSS OF MAIN RESERVOIR AIRRupture of the compressor discharge line would result inloss of pressure from the wet tank. When the pressure inthe wet tank (2) of the tractor unit falls below the warninglevel, due to a compressor failure or excessive leakage onthe tractor, the warning devices (11) will start to operate.In the diagram, the one-way check valve (9) has pre v e n t e dthe main reservoir air in the dry tank from escaping backto the wet tank and the ruptured line.

There is sufficient reservoir air pressure in the dry tank fora limited number of brake applications to stop the vehiclebefore the parking brakes are activated. (This will dependon how the parking brakes are piped.)

39


If this pressure is lowered to approximately 45-20 P.S.I. thetractor protection system will close automatically, placingthe trailer brakes into an emergency position.

The tractor protection system described in this manual is an example of a vehicle equipped with a type of cab-mounted trailer supply valve (24) which will close auto-matically when the pre s s u re in the supply line drops below45-20 P.S.I. The valve may also be closed manually.

MANUALLY OPERATEDTRAILER SUPPLY VALVESSome vehicles are equipped with a different type of cab-mounted trailer supply valve, which must be operatedmanually by the driver. It has two positions: NORMALand EMERGENCY.

The tractor will be equipped with a tractor protection valve,and the trailer unit with a relay emergency valve, as in theprevious system.

The functions of the trailer supply valve, tractor pro t e c t i o nvalve, and the relay emergency valve will be similar tothose illustrated. An important diff e rence is that the trailersupply valve must be shifted to the EMERGENCY positionmanually.

CHARGING THE TRAILER SYSTEM:

The driver places the trailer supply valve in the NORMALposition and main re s e rvoir air will be directed to thetrailer reservoir.

TRAILER BREAKAWAY:

The loss of emergency air in the supply line will cause thetrailer brakes to dynamite. To prevent air loss from thetractor, the driver must shift the trailer supply valve to the

EMERGENCY position. Otherwise the tractor air pressurewill bleed down and hold at 45-20 P.S.I.

RUPTURED SERVICE LINE:

As in the example, no action will occur until a brake appli-cation is made. Application of the brakes with a rupturedservice line will result in a loss of pressure in the system.When the main reservoir pressure drops to between 45-20P.S.I., the trailer brakes will dynamite.

RUPTURED SUPPLY LINE:

As in the example, the loss of supply pre s s u re will causethe trailer brakes to dynamite. To prevent air loss from thet r a c t o r, the driver must shift the cab-mounted trailer sup-p l y valve to EMERGENCY. Otherwise the tractor air pres-sure will bleed down and hold at 45-20 P.S.I.

If a slow loss of main reservoir air occurs, when the supplyline pressure drops to between 45-20 P.S.I., the relay emer-gency valve will cause the trailer brakes to dynamite.

MANUAL DYNAMITINGOF TRAILER:

Any time the driver shifts the cab-mounted trailer supplyvalve to the emergency position, and the trailer system ischarged, the trailer supply valve will exhaust the supplyline, dynamiting the trailer brakes.

The trailer brakes will remain applied only as long as airpressure remains within the trailer system. How long theair in the system will hold the brakes applied depends onhow airtight the system is. As a safety precaution, parkedtrailers should always be blocked to prevent a possiblerunaway.

To move a trailer that has been parked with the brakes inan emergency application, it is necessary to charge the sys-tem to release the trailer brakes.

40


SECTION SUMMARY1. What is the purpose of a two-way check valve?

2. Why should the glad hands be protected when not in use?

3. How can a driver control the trailer brakes independ-ently?

4. What are three ways of testing the emergency applica-tion of the trailer brakes?

5. Should the hand valve of a tractor trailer unit be usedfor parking?

6 . What is the main purpose of the tractor pro t e c t i o nv a l v e ?

7. What is the main purpose of the trailer supply valve?

8. Name three functions of the relay emergency valve?

9. Describe the function of the supply line.

10. Describe the function of the service line.

11. What will occur if the supply line ruptures?

12. What will occur if the service line ruptures?

13. What will occur if a brake application is made with aruptured service line?

14. If the foot valve and the hand valve are operated at thesame time, can the application pre s s u re be greater thanthe reservoir pressure?

N O T E S

41

Y U K O N A I R B R A K E M A N UA L S E C T I O N I V

N O T E S N O T E S

42


DUAL AIR SYSTEMS

A SIMPLE DUAL CIRCUIT

AIR SYSTEM

COMBINATION TRUCK

AND TRAILER

T-75 OFF-HIGHWAY

SYSTEM, SINGLE UNIT

L-75 OFF-HIGHWAY

SYSTEM, COMBINATION

WITH SCHEDULE X

TRAILER

SCHEDULE SX,

OFF-HIGHWAY TRAILER

WITH SPRING BRAKE

SECTION SUMMARY

QUESTIONS

SECTION V

43

Y U K O N A I R B R A K E M A N U A L S E C T I O N V

DUAL AIR SYSTEMSMore and more heavy duty vehicles on the road today areusing a Dual Circuit Air System. The system has been devel-oped to shorten stopping distances and reduce brake failure s .At first glance, the dual system might seem complicated,but if you understand the basic air system described so far,and if the dual system is separated into its basic functions,it becomes quite simple.

As its name suggests, the dual system is two systems or cir-cuits in one. There are different ways of separating the twop a rts of the system. On a two-axle vehicle, one circuit oper-a t e d the rear axle and the other circuit operates the frontaxle. If one system has a failure, the other system is isolatedand will continue to operate.

A SIMPLE DUAL CIRCUITAIR SYSTEM

NOTE: ALL PIPING DIAGRAMS ARE USED TO ILLUSTRATEBASIC DUAL CIRCUIT PRINCIPLES ONLY AND ARE NOTTO BE INTERPRETED AS REGULATIONS FOR, OR SPECIFI-CATIONS OF, DUAL AIR BRAKE SYSTEMS.

In the illustration below, air is pumped by the compressor(1) to the supply reservoir (2) which is protected from overpressurization by a safety valve (3). Pressurized air movesfrom the “wet” supply tank to the primary tank (green)and the secondary tank (red) through one-way checkvalves (9). At this point, the dual circuits start. Air from theprimary tank is directed to the foot valve (6). Air is alsodirected from the secondary tank to the foot valve. Thefoot valve is similar to the one described earlier in the SingleC i rcuit System, but has been divided into two sections

44

(two foot valves in one). One section of this dual foot valvecontrols the primary circuit and the other section controlsthe secondary circuit. When a brake application is made,air is drawn from the primary tank (green) through thefoot valve and is passed on to the rear brake chambers. Atthe same time, air is also drawn from the secondary tank(red), passes through the foot valve and is passed on to thefront brake chambers. If there is an air loss in either cir-cuit, the other circuit will continue to operate independ-e n t l y. Unless air is lost in both circuits, the vehicle willcontinue to have braking ability. The primary and sec-o n d a ry circuits are equipped with low pre s s u re warn i n gdevices (11) and pressure gauges (10).

DUAL CIRCUIT FOOT VALVE

S E C T I O N V Y U K O N A I R B R A K E M A N UA L

SIMPLE DUAL CIRCUIT SYSTEMWITH SPRING PARKING BRAKES

When spring brakes are added to a dual circuit system thesame type of dash control valve discussed previously is used.Blended air is used to supply the control valve (19). Blendedair is air taken from the primary and secondary circ u i t st h rough a two way check valve (22). With this pipingarrangement the vehicle can have a failure in either circuitwithout the spring brakes apply automatically. Unless air islost in both circuits the spring brakes will not apply.

SPRING BRAKES WITH MODULATOR VALVE

Spring type brakes in this system serve two purposes: first,as a parking brake, and second as an emergency system. Ifa failure occurs in the primary circuit (green), and a brakeapplication is made, control air from the foot valve is dire c t-e d to a spring brake modulator (27). As there is no supply

air to maintain balance in the modulator valve, due to theprimary circuit failure, the modulator valve then exhaustsair pressure from the spring brake circuit. The amount ofair released is equal to the amount of air applied by the footvalve. The release of air in the spring brake circuit causesthe drive axle to brake using spring brake pressure (18).When the brake is released, supply air from the secondarycircuit (red) returns the spring brakes to an off position.Brake applications can be repeated until all the air fromthe secondary circuit is lost, but, as the air pressure dropsbelow 85 P.S.I., the spring brakes won’t re t u rn to the full offposition, in fact they will start to drag. At approximately35 P.S.I., the spring brake control valve (19) on the dashexhaust the remaining air in the secondary circuit, and thespring brakes are fully applied. The only way the vehicle canbe moved after all air is lost is to repair the damaged circ u i tand recharge the system, or cage the spring brake system.

Tandem tractors without steering axle brakes will have thep r i m a ry and secondary systems split between the drivea x l e s.

45


1. Compressor2. Reservoir3. Safety Valve4. Drain Cock5. Governor6. Foot Valve7. Front Brake Chamber8. Rear Brake Chamber9. One Way Check Valve

10. Reservoir Pressure Gauge11. Low Pressure Indicator Switch12. Stop Light Switch14. Relay Valve

17. Automatic Limiting Valve18. Parking Brake (Spring)19. Parking Brake Control Valve20. Glad Hand Connector21. Hand Valve22. Two Way Check Valve23. Tractor Protection Valve24. Trailer Supply Valve26. Trailer Brake Chamber27. Spring Brake Modulator Valve28. Air Dryer30. Spring Brake Valve

46

COMBINATION TRUCKAND TRAILER WITHSPRING BRAKESThe trailer system is supplied by blended tractor air takenfrom the primary and secondary circuit through a twoway check valve as previously described.

The system is charged by opening the trailer supply valve(24), allowing air from the tractor to pass through the trac-tor protection valve (23), and the trailer spring brake valve(30) directly into the spring brake chambers (18). Whenair enters, the pressure protection part of the trailer springbrake valve opens, allowing the air to fill the trailer tanks.

When a brake application is made, blended control airacts on the relay valve (14) which releases air from thetrailer tank to the brake chambers.

In a dual circuit system, the primary circuit is protectedf rom air loss in the secondary circuit by the two-waycheck valve (22).

If the tractor breaks away from the trailer, the service andsupply lines will be pulled apart. The sudden loss of air inthe supply line will cause the tractor protection valve toclose, preventing air from escaping out of either broken

connection. The air supply in the tractor is sealed off andis available to control the tractor brakes. At the sameinstant, the sudden loss of air in the supply line causes thetrailer spring brake valve to exhaust the air from the trail-er spring brake chambers applying the trailer brakes. Thetrailer brakes cannot be released under these conditionsunless the lines are recoupled and the trailer tanks re c h a rg e d .

If only the supply line breaks between tractor and trailer,the same sequence of events will occur.

A break or rupture in the service line will not affect thetrailer until a brake application is made. A loss of pre s s u rein the tractor system will then result, causing the samee m e rgency brake application described above. However, thedriver will be able to release the parking brakes by releas-ing the foot valve, rebuilding air pressure and opening thetrailer supply valve.

To apply the parking brakes, the control valve (19) is closed,causing a loss of pre s s u re in the line which applies the springbrakes as described above.

To apply the parking brakes, the control valve (19) is closed,causing a loss of pre s s u re in the line which applies thespring brakes as described above.

The old and new tractor and trailer system are fully inter-changeable, whether they are dual system or single circuit,and whether they are systems with spring brakes or without.

N O T E : An air dryer (28) has been added to reduce theamount of moisture in the system.


T-75 OFF HIGHWAYSYSTEMS, SINGLE UNITAir from the compressor is pumped into the supply tank(blue). Pressurized air moves from the supply tank to thep r i m a ry tank (green), the secondary tank (red) and the fro n taxle (brown) through one way check valves (9). When the airp re s s u re reaches the governor cut-out setting (maximum125 P.S.I.), the compressor cuts out and is in its cooling or“unloaded” stage. When the air pre s s u re drops appro x i-mately 20 P.S.I., the governor (5) re t u rns the compressor toits pumping or “loading” stage.

When a brake application is made by pushing down the dualfoot valve treadle (6), air from the front axle circuit flowsto the front axle relay valve and air from the primary andsecondary circuits flows to the relay valves (14) at the rearaxles. This control pressure on the relay valves causes therelay portion of the valves to open, relaying air pressure tothe brake chambers. When the foot valve treadle is re l e a s e d ,air pressure in the control lines between the foot valve andrelay valves is released through the exhaust ports in thefoot valve. Air pre s s u re in the brake chambers is releasedthrough large exhaust ports in the relay valves.

Should either the primary or secondary SUPPLY circuit fail,the remaining circuit is isolated and continues to havebraking ability.

When either of the primary or secondary SUPPLY circ u i tfails, braking on the front axle and one rear axle will notbe affected due to the two way check valve (22) installedbetween the two service circuits.

Should a failure occur in the front axle circuit, the primaryor secondary circuit will not be affected.

NOTE: A failure in any of the circuits will activate the lowair warning system (11) and the driver MUST stop thevehicle to determine the cause. Should the air in the supplysystem drop approximately 35 P.S.I., the spring brakes willapply.

In this diagram the vehicle is equipped with a variable fro n twheel limiting valve (29). The application pressure deliv-ered to the front axle is determined by the setting made bythe driver.

To apply the spring parking brakes (18), the driver mustmove the control valve (19) to the “park” position. Thisexhausts the control pressure from the spring brake relayvalve (30) which then releases the air from the spring brakes,allowing them to apply.

To release the spring parking brakes, the driver must movethe control valve to the “release” position. This directs airto the spring brake relay valve, which supplies air pressureto the spring brakes, which compresses or “cages” thesprings and releases the brakes.

47


L-75 OFF HIGHWAYSYSTEM COMBINATIONWITH SCHEDULE XTRAILERThe L-75 tractor brakes are piped similar to the T-75 system.A low pressure governor (31), a dump valve (32), a handvalve (21), a two-way check valve (22), and a relay typetractor protection valve (33), have been added. In this sys-tem, when air pressure rises to approximately 60 P.S.I., thelow pressure governor opens, allowing air pressure to closethe emergency dump valve. When maximum air pressureis reached, the governor unloads the compressor.

To fill the trailer tanks, the driver must open the trailersupply valve (24) until the trailer supply line gauge (34)shows over 50 P.S.I. The trailer supply valve will then re m a i nopen on its own, keeping the trailer tanks at the same pre s-s u re as the tractor tanks.

When a brake application is made with the foot valve, airflows through a two-way check valve (22) to the controlp o rt on the tractor protection relay valve (33), whichrelays air pressure from the supply line to the service line.The service pressure line relays air from the trailer tankst h rough the relay emergency valves (25) to the trailerbrake chambers.

When the brakes are released, air pressure in the serviceline is exhausted through the exhaust port in the tractorp rotection relay valve. Air pre s s u re in the trailer brake cham-b e r s is exhausted through a large exhaust port in the relaye m e rgency valves. This action, both in application andrelease, is simultaneous on tractor and trailer.

To apply the trailer brakes only, or for a higher brake appli-cation on the trailer than on the tractor, the driver applieshand valve (21), air from the tanks flows through the handvalve into a double-check valve (22), to the tractor protec-tion relay valve (33). Brakes on the trailer are then appliedand released as described above.

If necessary, the trailer brakes can be “dynamited” by clos-ing the trailer supply valve. This shuts off the air supplyfrom the tanks and simultaneously exhausts the supplyline into the atmosphere. Loss of air pre s s u re in the supplyline causes the trailer’s relay emergency valves to dumpfull reservoir pressure into the brake chambers.

The trailer system will provide a rapid application andrelease of the trailer brakes as each axle has its own airtank and relay emergency valve.

If a brake chamber or air line ru p t u res on one axle, theother axle continues to have braking ability for emergencystops.

The L-75 tractor system has the same air loss safety feature sas the T-75.

SCHEDULE SX OFF HIGHWAY TRAILERWITH SPRING BRAKESThe SX trailer brake system is piped similar to the ScheduleX system. Both system provide air loss safety features. Aone-way check valve (9), a third tank (2), a spring brakecontrol valve (19), two four-line two-way check valves (35)and four spring brakes (18) have been added.

48


The addition of spring brakes serves two functions. It pro-vides a method of securing the trailer when parked and actsas an emergency brake for the trailer axle that has lost itssupply re s e rvoir pre s s u re. (The trailer spring brakes are heldoff or caged by trailer supply reservoir pressure.)

If the trailer has lost all supply reservoir pressure and mustbe moved, the spring brakes may be released by openingthe spring brake control valve (19) located at the rear ofthe trailer. This directs air from the isolated third tank torelease the spring brakes. The spring brakes may be resetby closing the spring brake control valve. Care should betaken when moving the trailer as the service brakes willnot apply.

N O T E : Regulation piping diagrams for off highway air brakesystems are listed in Appendix I pursuant to the Highway(Industrial) Act.

SECTION SUMMARY1. What is the basic principle of the dual circuit system?

2. What valve is used to protect the primary circuit fromthe secondary circuit?

3. In a simple dual circuit system, will the vehicle continueto have braking ability if one circuit fails?

4 . Is there a diff e rence between the foot valve used in asingle circuit system and the foot valve used in the dualcircuit system?

5. Name two functions of the spring brakes in a dual circ u i tsystem.

6. Describe the functions of the spring brake modulatorvalve.

7. If the trailer breaks away from the tractor on a dual circuit highway system, what applies the brakes on the t r a i l e r ?a) the trailer reservoir pressureb) the spring brakes

8. What is blended air?

9 . Can a single circuit trailer be towed with dual circ u i ttractor?

10. Name the components found on an L-75 system thatare not on a dual circuit “highway” system.

11. If the trailer breaks away from the tractor on an L-75system, what applies the trailer brakes?a) the trailer reservoir pressureb) the spring brakes if equipped

N O T E S

49

Y U K O N A I R B R A K E M A N UA L S E C T I O N V

N O T E S N O T E S

50


51

SECTION VIPRE-TRIPINSPECTION

SINGLE UNIT

COMBINATION UNIT

BRAKE ADJUSTMENT

AIR OPERATED

DISC BRAKES

IN SERVICE CHECK

MAINTENANCE

AND SERVICING

OF AIR BRAKES

SECTION SUMMARY

QUESTIONS

Y U K O N A I R B R A K E M A N U A L S E C T I O N V I

PRE-TRIP INSPECTION

PROCEDURE FOR AIR PRE-TRIP, SINGLE UNIT •SINGLE OR DUAL CIRCUITCheck vehicle on level ground if possible. Set parkingbrake, block wheels, ensure that all tanks are drained.Suggested maximum time required for this pre-trip inspec-tion is 15 minutes.

CIRCLE CHECK1. Check security and condition of compressor and belts

under hood.2. Check condition and security of hoses and flex lines.3. Pull all slack adjusters manually (use pry bar if neces-

sary). Check for travel, mechanical condition, wear.4. On spring brake-equipped vehicles, check that angle of

pushrod and slack adjuster arm does not exceed 90˚.NOTE: A brake adjustment (“S” cam type only) is requiredas part of the Air Brake Pre-trip Examination.

BUILD UP AIR PRESSURE, CHECK THAT:1. Warning device(s) cuts out at approximately 60 P.S.I.;2. Pressure builds from 50-90 P.S.I. within 3 minutes;3 . G o v e rnor cuts out between 105 P.S.I. minimum and 125

P.S.I. maximum.

AT MAXIMUM PRESSURE:1. Release parking brakes (if equipped) to prevent com-

pounding;2. Shut off engine;3. Make full brake application. Check for:

a) air loss less than 3 P.S.I. within 1 minute;b) audible air leaks.

START MOTOR, FAN BRAKES1. Stop fanning at 80 P.S.I. to see if governor has cut in

(needle should start to climb).2. Continue fanning:

a) warning device(s) should start operating at approxi-mately 60 P.S.I.;

b) parking brake should apply at 45 - 20 P.S.I. if plumbedinto main reservoir tanks.

REBUILD PRESSURE TO MAXIMUM1. With parking brakes set, remove wheel chocks2. Gently tug test against parking brakes in low gear.3. Release parking brakes.4. Move vehicle ahead slowly and make a service brake

application to check brake response.

PROCEDURE FOR AIR PRE-TRIP, COMBINATIONUNIT • SINGLE OR DUAL CIRCUITCheck vehicle on level ground if possible. Set parking brake,block wheels, ensure that all tanks are drained.

Suggested maximum time required for this pre-trip inspec-tion is 30 minutes.

NOTE: Draining of trailer tanks is not required for the AirBrake Pre-trip Examination.

CIRCLE CHECK1. Check security and condition of compressor and belts

under hood.2. Check condition and security of hoses and flex lines.3. Pull all slack adjusters manually (use pry bar if neces-

sary). Check for travel, mechanical condition, wear.4. On spring brake-equipped vehicles, check that angle of

pushrod and slack adjuster arm does not exceed 90˚.NOTE: A brake adjustment (“S” cam type only) is requiredas part of the Air Brake Pre-trip Examination.

BUILD UP PRESSURE, CHECK THAT:1. Warning device(s) cuts out approximately 60 P.S.I.2. Pressure builds from 50 - 90 P.S.I. within 3 minutes;3. Governor cuts out between 105 P.S.I. minimum and

125 P.S.I. maximum.4. Release spring brakes.5. Charge trailer, rebuild pressure if necessary.

BREAK SUPPLY LINE:1. Trailer brakes should apply;2. There should be no air loss from trailer;3. Air from tractor should:

a) stop immediately (mandatory on L75) or,b) bleed down to between 45-20 P.S.I. and then stop.

NOTE: Under no circumstances should tractor pressuredrain below 45-20 P.S.I.

4. Reconnect supply line and re c h a rge trailer, rebuild pressure if necessary.

BREAK SERVICE LINE:1. No air should escape.2. Make full brake application:

a) trailer brakes should apply immediately and thereshould be no further loss of air from the service line (mandatory on L75), or,

b) trailer air should bleed down to between 45-20 P.S.I.and then service brakes will apply and there shouldbe no further loss of air from the service line.

3. Reconnect service line, charge trailer and rebuild pres-sure to maximum reservoir pressure.

AT MAXIMUM PRESSURE:1. Release parking brakes (if equipped) to prevent com-

pounding;2. Shut off engine;3. Make full brake application. Check for:

a) air loss less than 4 P.S.I. within 1 minute (6 P.S.I. with 2 trailers)

b) audible air leaks.

52

S E C T I O N V I Y U K O N A I R B R A K E M A N UA L

START MOTOR, FAN BRAKES1. Stop fanning at 80 P.S.I. to see if governor has cut in

(needle should start to climb).2. Continue fanning:

a) warning devices should start operating at approxi-mately 60 P.S.I.;

b) trailer brakes should apply at 45-20 P.S.I (L75 must apply at 50 P.S.I.);

c) parking brake should apply at 45-20 P.S.I. if plumbedinto main reservoir tanks.

REBUILD PRESSURE TO MAXIMUM1. With parking brakes set, remove wheel chocks..2. Gently tug test against parking brakes in low gear.3. Release parking brakes and set trailer brakes with the

hand valve.4. Gently tug test against trailer brakes in low gear.5. Release trailer brakes.6. Move vehicle ahead slowly and make a service brake

application to check brake response.

BRAKE ADJUSTMENTAll drivers should be trained to make brake adjustments.If a driver must adjust the brakes on the vehicle (at amandatory brake check sign for example), the followingsteps are recommended for “S” cam brakes:

1. Block the wheels, release spring brake and shut off theengine, leaving it in low gear. Check the push rod travel by pulling on a slack adjuster by hand or with a pry bar.If on common brake chamber, slack (push rod travel)exceeds 3/4 inch brake adjustment is required.

CAUTION: A strong physical pull is necessary to move allthe slack out of the travel.

2. After determining that brake adjustment is necessary:a) Use a proper wrench to disengage the external lock-

ing device (if so equipped) from the adjustment boltof the slack adjuster.

b) Turn the adjustment bolt until the lining contacts the drum. If possible, visually check to see that thebrake linings are in contact with the brake drum.

c) When turning the adjustment bolt on the slackadjuster, the spline of the “S” cam shaft should turnin the same direction as if a brake application werebeing made.

d) Back the adjustment bolt about 1/4 to 1/2 of a turn.This should result in proper adjustment.

e) Now re-check for free play of slack adjuster travel.The travel should now be between 1/2 and 3/4 of an inch. If travel is checked by watching the push rodwhile someone makes a brake application, travelshould not exceed 1 1/2 inches.

NOTE: Please see page 15 for automatic slack adjusters.

AIR OPERATED DISC BRAKES

These brakes require special adjustment to the manufac-turer’s specifications.

ALL BRAKES

At the first stop after adjusting the brakes, check eachbrake drum or rotor for temperature. An extra hot brakedrum indicates that you have adjusted it too tightly.

Only when all brakes are properly adjusted will the systembe able to absorb the required amount of energy to bringthe loaded vehicle to a safe stop under extreme condi-tions. Remember that poorly adjusted air brakes mayappear to be working at slow speeds on light air applica-tion. THE ONLY WAY TO BE SURE THAT YOUR VEHICLEBRAKES ARE PROPERLY ADJUSTED IS TO PHYSICALLYCHECK THE BRAKES AT EACH WHEEL.

IN SERVICE CHECKSSTEEP DOWNGRADE: In some areas of the Province, signssuch as the above are posted in advance of steep or longdowngrades.

Such signs indicate that the driver must stop the vehiclein the pull-out area and inspect the vehicle’s braking sys-tem before proceeding. Check:

1. Compressor is maintaining full reservoir pressure.

2. Push rod travel is within limitations on all chambers.

3. No audible air leaks.

4. Glad hands and lines are secure.

5. Drums, bearings and tires are not overheating.

6. Trailer supply valve is operating properly.

The driver must be aware of the condition of the vehicle’sbraking system at all times. This can be done by:

1. SEEING the pressure gauges;

2. HEARING the warning signals; and

3. FEELING the braking response of the vehicle.

By these methods of observation, the driver should be ableto notice any defects developing in the braking systemand be aware that service or adjustment are required.

Ensuring that the vehicle is in proper condition to drive isthe responsibility of everyone involved. The extent of thed r i v e r ’s responsibility to make repairs will depend on manyfactors, such as:

53


The maintenance policy of the company;The driver’s mechanical experience;Whether or not the vehicle is operator owned; and,The availability of repair service.

MAINTENANCE AND SERVICING OF THEAIR BRAKE SYSTEM

PLACING THE VEHICLE IN SERVICE

Before placing a vehicle into service, specific items mustbe checked to ensure the vehicle is in satisfactory condition.Preventative maintenance checks are designed to preventmechanical failures which could result from neglect. Thesechecks cut repair costs by detecting minor mechanical defectswhich would otherwise develop into major breakdowns!

Items to be checked include:• Cooling system level and condition• Fan and compressor drive belts• Crankcase oil level and leaks• Batteries, electrolyte level• Fuel tanks and gauges• Tire and wheels• Lights, signals, horn• Windshield, wipers, mirrors• Steering• Emergency equipment• BRAKES, service and park

See Professional Driving Guide for more details

P reventative maintenance may be assigned to a mainte-nance crew or to the driver, but one person alone is finallyresponsible to ensure that the braking system is in operatingcondition before the vehicle moves. THE DRIVER ISRESPONSIBLE.

SECTION SUMMARY1. What is the maximum time permitted for the compre s s o r

to build from 50 P.S.I. to 90 P.S.I?

2. What is the maximum pressure loss permitted on a fullbrake application with the motor shut off?

3. How can the holding power of the trailer brakes be tested?

4. Should all drivers be able to adjust “S” cam and drum braking systems?

5. What is the final brake test that should be made beforethe vehicle is put into service?

6. What tests must be made at a “truck stop here” sign posted before a steep or long downgrade?

7. What are three ways to check the condition of a vehi-cle’s braking system?

8. Is a brake adjustment part of the air brake test given by the Motor Vehicle Branch?

9. Do the brake adjustment specifications differ between the “S” cam and drum system and the disc brakes system?

1 0 . When should the brakes be checked after an adjustmenthas been made?

METRIC CONVERSION TABLEMany vehicles equipped with air brakes have gauges meas-uring air pressure in kilopascals (kPa.)

kPa to P.S.I P.S.I to kPa

5 .................... 0.7252 1 ....................... 6.8947

10 .................... 1.4504 2 ..................... 13.7894

15 .................... 2.1756 3 ..................... 20.6841

20 .................... 2.9008 4 ..................... 27.5788

25 .................... 3.6260 5 ..................... 34.4735

30 .................... 4.3512 6 ..................... 41.3682

35 .................... 5.0764 7 ..................... 48.2629

40 .................... 5.8016 8 ..................... 55.1576

45 .................... 6.5268 9 ..................... 62.0523

50 .................... 7.2519 10 ..................... 68.9470

60 .................... 8.7023 15 ................... 103.4205

70 .................. 10.1527 20 ................... 137.8940

80 .................. 11.6031 25 ................... 172.3675

90 .................. 13.0535 30 ................... 206.8410

100 .................. 14.5039 35 ................... 241.3145

150 .................. 21.7558 40 ................... 275.7880

200 .................. 29.0078 45 ................... 310.2615

250 .................. 36.2597 50 ................... 344.7350

300 .................. 43.5117 55 ................... 379.2085

310 .................. 44.9621 60 ................... 413.6820

350 .................. 50.7636 65 ................... 448.1555

400 .................. 58.0156 70 ................... 482.6290

415 .................. 60.1912 75 ................... 517.1025

450 .................. 65.2675 80 ................... 551.5760

500 .................. 72.5195 85 ................... 586.0495

550 .................. 79.7714 90 ................... 620.5230

585 .................. 84.8478 95 ................... 654.9965

600 .................. 87.0234 100 ................... 689.4700

650 .................. 94.2753 105 ................... 723.9435

700 ................ 101.5273 110 ................... 758.4170

725 ................ 105.1532 115 ................... 792.8905

750 ................ 108.7792 120 ................... 827.3640

800 ................ 116.0312 125 ................... 861.8375

850 ................ 123.2831 130 ................... 896.3110

900 ................ 130.5350 135 ................... 930.7845

950 ................ 137.7870 140 ................... 965.2580

1000 ................ 145.0389 145 ................... 999.7315

1050 ................ 152.2909 150 ................. 1034.2050

54

S E C T I O N V I Y U K O N A I R B R A K E M A N UA L

55

N O T E S N O T E S


56

N O T E S N O T E S

ISBN 1-55018-634-5 PRINTED IN CANADA

airbrake manual english

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