air brakes manual

7/29/2019 Air Brakes Manual

1/95

Air Brake ManualWorking with driversto make our roads safer.


2/95

Air Brake Endorseme Permitstheholdertodrivevehicles

equippedwithairbrakesinclassof

vehicleforwhichthedriverislicenc

Toadjustmanualorautomaticslack

adjusters,theoperatormustholdan

brakeendorsement.

Requirements for AiBrake Endorsement MustcompleteanAirBrakeWritten

MustcompleteanAirBrakePractica

Anairbrakeendorsementisnotrequi

foralicensedholdertodriveaClass3airbrakeequippedvehicleifitisregis

asafarmvehicle.Class3or5airbrake

equippedvehicles,fittedwithdealer

plates,maybedrivenbyfarmersfor

demonstrationpurposes.

Use of Licence asa Learner PermitAholderofanylicencemaybeauthor

tooperatevehiclesequippedwithairb

asalearneraftermeetingtherequired

writtenstandards.Thelearnermustb

supervisedbysomeonewhohasheld,

atleasttwoyears,anairbrakeendors

ForewordheAirBrakeManualhasbeenprepared

yManitobaPublicInsurance,toassist

riversinunderstandingthebasicoperation

ndfunctionofanairbrakesystem.

hestudyofthismanual,togetherwith

racticalinstruction,isrecommended

oradriverwhoispreparingfortheair

rakeexamination.Alargeillustrationof

completedualairbrakesystemislocated

ntheinsidecoverandcanbefoldedout

ndreferredtowhenstudyingthismanual.

tudyquestionsareincludedattheendof

achsectionsothatreadersmayselftest

heirunderstandingofthesubjectmatter.

riverswhohavequalifiedandareauthorized

ooperateairbrakeequippedvehiclesarencouragedtoreviewthismanualona

eriodicbasistoensuretheyarefully

wareofthepropermethodofinspecting

nairbrakesystemandidentifyingproblems

hatcanoccurwhenthesystemmalfunctions.

heillustrationsandexplanationsofvarious

ypesofbrakesystemdesignsareprovided

orinstructionalpurposesonly.Mostair

augesmeasureinimperialunits.Therefore

hemeasurementsusedandrelatingtothe

rbrakesystemwillbeinimperialunits.

hismanualhasnolegislativesanction.

orinterpretingandapplyingthelaw,

onsultThe Drivers and Vehicles Act,The Highway

raffic Actanditsregulations.Salesofthis

manualarefinalandnotrefundable.

Wegratefullyacknowledgethecontributionsof

ljurisdictions,particularlyBritishColumbia.


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Dual Air Brake System

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

16

17

6

18 15141920212211 122324252628

2729

33

30

6 6

1. Compressor

2. Governor

3. Air dryer

4. Safety valve

5. Supply/wet reservoir

6. Drain valves7. Oneway check valves

8. Primary/dry reservoir

9. Low pressure indicator

10. Secondary/dry reservoir

11. Rear service brake chambers

12. Spring parking brake chambers

13. Tractor relay valves

14. Trailer service brake chamber

15. Trailer spring parking brake chamber

16. Trailer reservoirs

17. Trailer relay valve

18. Trailer spring brake valve

19. Anticompound lines

20. Glad hands

21. Supply (emergency) line

22. Control (service) line

23. Spring brake modulator v

24. Tractor protection valve

25. Stop lamp switch

26. Twoway check valves27. Spring parking brake cont

28. Trailer supply valve

29. Reservoir air pressure gau

30. Trailer brake hand valve

31. Foot valve

32. Front service brake cham

33. Quick release valve

34. Automatic front brake lim

This illustration has an autom

brake limiting valve (34), and t

the control valve (35), for a ma

brake limiting valve (36) are no

here, but appear later in the m

Legend

bluesupply/

greenprima

redsecondar

yellowspring

brake system

dark greentr

Incab portiohighlighted


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CHECK THE SLACK!

It is up to YOU,the DRIVER,

to ensure that

your vehicle hassafe, properly

adjusted brakes.


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Air Brake Manual

Table of ContentsForeword Foldot

AirBrakeEndorsement Foldot

RequirementsforAirBrakeEndorsement Foldot

UseofLicenceasaLearnerPermit Foldot

DualAirBrakeSystemIllustrationFoldot

HowtoBookYourTestAppointment 3

KeyPointsAboutYourTests 4

Section 1 Brakes and Braking 5

HeatEnergyTractionFriction 6

SpeedWeightDistance 7

BrakingForce 7

UseofAirPressure 8

LeverageandAirPressure 8

StoppingDistance 8

SectionSummaryQuestions 9

Section 2 The Components of an Air Brake System 11

TheComponentsofanAirBrakeSystem 12CompressorandGovernor 12

Reservoirs 14

AirDryer 15

SafetyValve 16

FootValve 16

BrakeChambers,SlackAdjustersandBrakeLining 16

WedgeBrakes 20

DiscBrakes 21

AirOverHydraulicBrakeSystems 21

AirActuatedHydraulicBrakeSystem 21

AirBoostHydraulicBrakeSystem 22


Section 3 How the Basic System Works 25

BasicAirBrakeSystem 26

OnewayCheckValve 26

AirPressureGauge 27

BrakeApplicationGauge 27

LowPressureWarningDevice 27

StopLightSwitch 27

QuickReleaseValve 28

RelayValve 28

ManualFrontBrakeLimitingValve 28

AutomaticFrontBrakeLimitingValve 29

TandemRearAxles 30


Section 4 Spring Parking Brakes

SpringParkingBrakeSystems

UsingaSpringParkingBrake

MechanicalRelease(Caging)

SectionSummaryQuestionsSection 5 Trailer System

GladHands

ApplicationLine

TrailerBrakeHandValve

TwowayCheckValves 4

TractorProtectionSystem

TractorProtectionValve

TrailerSupplyValve

AutomaticTrailerSupplyValveSystem 4

TractorandTrailerCoupled 4

ChargingtheTrailerSystem

FootorHandValveBrakeApplicationEmergencyApplication 4

Supply(Emergency)LineRupture

Control(Service)LineRupture

LossofReservoirAirPressure

ManualTrailerSupplyValve

TrailerSpringParkingBrakes

SectionSummaryQuestions

Section 6 Dual Air Brake System

BasicDualAirBrakeSystem

DualAirBrakeSystemwithSpringParkingBrakes

SpringParkingBrakeswithModulatorValve

CombinationTractorandTrailer withSpringParkingBrakes


Section 7 Electronic Controlled Brakingand Traction Systems 6

AntilockBrakeSystem(ABS)

AutomaticTractionControl(ATC)



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2 Air Brake Manual

Section 8 Brake Adjustment and In-Service Check 65

BrakeAdjustment 66

BrakeAdjustmentItsCritical 66

BrakeAdjustmentItstheLaw 66

Definition 67

CheckingandAdjustingSCamBrakes

ManualSlackAdjusters 69PryMethodofFreeStrokeMeasurement 69

AppliedStrokeMethod(servicebrakeapplication) 70

BrakeAdjustmentIndicators 70

BrakeAdjustmentManualSlackAdjuster 71

Adjustment 71

AdjustingS-CamBrakeswithAutomaticSlack

AdjustersWhyWhenHow 72

EmergencyManualBrakeAdjustment

AutomaticSlackAdjusters 73

DiscBrakeAdjustment 75

WedgeBrakeAdjustment 75

AfteraBrakeAdjustment 75

In-ServiceChecks 76MaintenanceandServicingofthe

AirBrakeSystem 76

AirBrakeAdjustmentMyths 77


Section 9 PreTrip Air Brake Inspection 79

VehicleInspection 80

AirBrakePracticalTest 80

PreTripInspectionCombinationUnit 81

BrakeAdjustmentCombinationUnit 82

PreTripInspectionSingleUnit 85

BrakeAdjustmentSingleUnit 86PreTripInspectionAirOverHydraulic

(AirActuated)BrakeSystem 88


MetricConversionTable 89

Pre-tripInspectionTearOutSheet 91

OrganandTissueDonation 93


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Air Brake Manual

Test Appointments

How to Book Your Knowledge or Road Test

Visit any Autopac agent* to pay for your test and to

schedule your test appointment.

The agent can provide you details of available

appointments at any testing location.

You can schedule your test appointment up to eight

weeks in advance.

Both knowledge and road tests are conducted during

weekdays only (with the exception of holidays) at most

locations. Saturday appointments are available in

Winnipeg at Barnes Street, Lexington Park and Main

Street locations.

Test Locations Winnipeg

15 Barnes Street (at Bison Drive)

40 Lexington Park (at Gateway Road)

1284 Main Street

930 St. Marys Road (French Language Services available)

125 King Edward Street East

1103 Pacic Avenue

420 Pembina Highway

Test Hours

BarnesStreet,LexingtonParkandMainStreet:

MondayFriday7:30a.m.5:30p.m.

Saturday8:30a.m.4p.m.

Allotherlocations:

MondayThursday7:30a.m.5:30p.m.

Friday7:30a.m.4:30p.m.

(Closeat4:30p.m.MondayFridayduringJune,

JulyandAugust.)

Test Locations Outside Winnipeg

Arborg|323SunsetBoulevard

Beausejour|848ParkAvenue

Brandon|7311stStreet

Dauphin|217IndustrialRoad

Portage la Prairie|25TupperStreet

Selkirk|1008ManitobaAvenue

Steinbach|165ParkRoadWest,Unit2,

ClearspringVillageMall

The Pas|424FischerAvenue

Thompson|53CommercialPlace

Winkler|355BoundaryTrail

Test Hours

Alllocations:

MondayFriday8:30a.m.4:30p.m.

Mobile Test Units

Mobile Test Units serve rural customers living outside

the above locations. Mobile Test Unit locations and

schedules are available from any Autopac agent.

*For a list of Autopac agents, see mpi.mb.ca.

Key Points About Your Tests

Cost*

Aknowledgetestcosts$10.

Costsforroadtests:

Class1licence:$50 Class5licence:$30 Class2or3licence:$45 Class6licence:$30

Class4licence:$35 AirBrake:$30**

*Costmaychange

**Thereisnoadditionalchargefortheairbraketest

ifyouaretakingaroadtestforaclass1,2,3,4or5

licenceandthevehicleyouareusingforthetestis

equippedwithairbrakes

General Testing Information

Thereisa30-minutetimelimittocompletea

knowledgetest.

Onlyoneknowledgeorroadtestofthesameclassmbecompletedperday.

Thereisawaitingperiodbetweentheknowledgeand

roadtestforsomeclassesoflicences.

IfyouareintheGraduatedDriverLicensing(GDL)

programandarenotsuccessfulonyourknowledge

test,thereisaseven-daywaitingperiodbeforeyou

maytakethetestagain.

IfyouareinGDLprogramandarenotsuccessfulon

yourroadtest,thereisa14-daywaitingperiodbefore

youmaytakethetestagain.

Theknowledgetestisnotanopenbooktest.Cellularphonesandelectronicdevicesarenotallowedinthe

testarea.

Pleasecheck-in15minutespriortoyourscheduled

appointmenttime.

Once youve made your test appointment at anAutopac agent, you can call your agent or the followinnumbers for additional information or assistance:

InWinnipegcall2049857000

OutsideWinnipegcall18006652410


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Air Brake Manual

BrakesandBraking

Section1


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6 Air Brake Manual

HeatEnergyTractionFriction

Foravehicletomovealongthehighway,aninternal

combustionenginemustconvertitsheatenergyinto

mechanicalenergy.Thismechanicalenergygoes

fromtheenginetothedrivingwheeltiresbymeans

ofasystemofconnectingrods,shaftsandgears.

Thefinalfactorthatmovesthevehicleistheamountoftractionitstireshaveontheroadsurface.

Frictionistheforcethatresistsmovementbetweentwo

surfacesincontactwitheachother.Tostopavehicle,

thebrakeshoeliningsareforcedagainstthemachined

surfacesofthebrakedrums,creatingfriction.

Thisfrictionproducesheat.

Theengineconvertstheenergyofheatintotheenergy

ofmotion;thebrakesmustconvertthisenergyof

motionbackintotheenergyofheat.Thefriction

betweenbrakedrumsandliningsgeneratesheatwhile

reducingthemechanicalenergyoftherevolvingbrake

drumsandwheels.Theheatproducedisabsorbedby

themetalbrakedrums,whichdissipatetheheatinto

theatmosphere.Theamountofheatthebrakedrums

canabsorbdependsonthethicknessofthemetal.

Whenenoughfrictioniscreatedbetweenthebrake

liningandthedrums,thewheelsstopturning.

Thefinalfactorthatstopsthevehicleisthetraction

betweenthetiresandtheroadsurface.

Ifa200horsepowerengineacceleratesavehicle

to100km/hinoneminute,imaginethepower

neededtostopthissamevehicle.Also,considerthat

thevehiclemighthavetostopinanemergencyin

aslittleassixseconds(just1/!)thetimeittook

toreach100km/h).

Tostopthevehiclein1/!)thetimeittookto

acceleratewouldrequireastoppingforceof10

timestheaccelerationforcetheequivalentofapproximately2,000horsepower.Ifthevehiclehad

sixwheels,eachwheelwouldhavetoprovide1/^the

brakingforce.Ifoneortwoofthewheelshadbrakes

thatwerenotproperlyadjusted,theotherwheels

wouldhavetodomorethantheirshareofthebraking,

andthatmightbemorethantheirbrakeswere

constructedtostand.Excessiveuseofthebrakeswould

thenresultinabuildupofheatgreaterthanthebrake

drumscouldabsorbanddissipate.Toomuchheat

resultsinbrakedamageandpossiblefailure.

Mostbrakeliningsoperatebestataround250Cand

shouldnotexceed425C.Itsimportanttounderstandthatthepowerneededtostopgeneratesheatwhich

coulddamagethebrakes.

100 km/h

10X

Brake Drums

250CNormal

1100CPanic!

425CMaximum


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Air Brake Manual

SpeedWeightDistance

Thedistancerequiredtostopavehicledepends

onitsspeedandweight,inadditiontoenergy,

heatandfriction.Thebrakingforcerequiredto

stopavehiclevariesdirectlywithitsweightand

speed.Forexample,iftheweightisdoubled,the

brakingforcemustbedoubledtobeabletostopinthesamedistance.Ifthespeedisdoubled,

thebrakingforcemustbeincreasedfourtimes

tobeabletostopinthesamedistance.When

weightandspeedarebothdoubled,thebraking

forcemustbeincreasedeighttimestobeableto

stopinthesamedistance.

Forexample,avehiclecarryingaloadof14,000kg

at16km/hisbroughttoastopin30metreswith

normalapplicationofthebrakes.Ifthissame

vehiclecarried28,000kgat32km/h,itwould

requireeighttimesthebrakingforcetostopthe

vehiclein30metres.Thiswouldbemorebraking

forcethanthebrakescouldprovide.Novehiclehas

enoughbrakingforcewhenitexceedsitslimitations.

Braking Force

Mechanical

Brakingsystemsusedevicestogainamechanical

advantage.Themostcommondeviceforthis

purposeisleverage.Aleverisplacedonapivot

calledthefulcrum.AsthedistancefromAtoC

isfourfeet,andfromCtoBisonefoot,theratio

isfourtoone(4:1).Forcehasbeenmultipliedby

theleverageprinciple.

Lookatthissimpleleversystem:

Ifa100lbdownwardforceisappliedatpointA,then

theupwardforceatpointBis400lb.

B = 400 lb

C

A = 100 lb

C

A100 lb

Scam brake

B400 lb

B400 lb

A B

4 eet 1 oot

C

Applied orce= 100 lb

Delivered o= 400 lb

A

BC


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8 Air Brake Manual

Use o Air Pressure

Forcecanalsobemultipliedbytheuseofairto

gainfurthermechanicaladvantage.Everyonehas

felttheforceofaironawindyday.Aircanbe

compressed(squeezed)intoamuchsmallerspace

thanitnormallywouldoccupy,forinstance,

aircompressedintirestosupporttheweightofavehicle.Thesmallerthespaceintowhichairis

squeezed,thegreatertheairsresistancetobeing

squeezed.Thisresistancecreatespressure,which

isusedtogainmechanicaladvantage.

Ifaconstantsupplyofcompressedairisdirected

throughapipethatisoneinchsquare,andifa

oneinchsquareplugwereplacedinthepipe,

thecompressedairwouldpushagainsttheplug.

Ascalecanbeusedtomeasurehowmanypounds

offorcearebeingexertedbytheairagainsttheplug.

Ifthescaleregisters10pounds,forexample,then

itcouldbesaidtheforceis10poundsontheone

squareinchsurfaceoftheplugor10poundsper

squareinch(psi).

Themorecompressedtheairinthesupplyreservoir,

thegreatertheforceexertedonthefaceoftheplug.

Leverage and Air Pressure

Inactualoperation,pipesareroundandplugsare

diaphragmsofflexiblematerialactingagainstpush

rods.Ifcompressedairof120psiactsonadiaphragm

of30squareinches,3,600lbofforceisproduced

(120x30).Applythisforcetoapushrodtomove

a6inchslackadjusteroperatingacamandthetotalforceequals21,600inchpoundstorque(3,600x6),

or1,800footpoundstorque(21,60012).Itrequires

25to30footpoundsoftorquetotightenthewheel

onacar.Thiscomparisonillustratestheforce

obtainedfromusingmechanicalleverageandair

pressurecombined.

Stopping Distance

Stoppingdistanceconsistsofthreefactors:

driversreactiontime

brakelag

brakingdistance

Driversreactiontime:Reactiontimeisoften

calledthinkingtime.Thetimeittakesfrom

themomentahazardisrecognizedtothetime

thebrakeisapplied,approximately Pofasecond.

Brakelag:Asairishighlycompressible,itrequires

arelativelylargevolumeofairtobetransmitted

fromthereservoirtothebrakechamberbefore

thereisenoughpressureforthebrakestoapply.

Itcanbesaidthatbrakelagisthetimeittakes

theairtotravelthroughaproperlymaintained

airbrakesystem(approximately 4/!)ofasecond).

1 squareinch

10 psi

30 square inches

120 psi

6 inches

1inch


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Air Brake Manual

Passengercar

Brakes applied

Loadedtruck

Actual stop

Actual stop

Brakingdistance:Theactualdistancethevehicle

travelsafterthebrakeisapplieduntilthevehiclestops.

Thedistancedependsontheabilityofthebrakelining

toproducefriction,thebrakedrumstodissipateheat

andthetirestogriptheroad.

Driversshouldnevertaketheirbrakesforgranted.

Thebrakingsystemmustbetestedandtheadjustmentcheckedbeforeplacingthevehicleintoservice.Drivers

mustunderstandthebrakingsystem,realizeits

capabilitiesandlimitations,andlearntousethem

tothebestadvantage.

Heavyvehiclesrequirepowerfulbrakingsystems

thatareobtainedbyuseofmechanicalleverageand

airpressure.Brakesmustbeusedkeepinginmind

theheatgeneratedbyfriction.Iftheheatbecomes

toogreat,brakingeffectivenesswillbelost.Theheavier

theloadandthefasterthespeed,thegreatertheforce

neededtostop.

Itisimportanttorememberthatanairbrakeequippedvehicle,evenwithproperlyadjustedbrakes,willnot

stopasquicklyasapassengercar.

Comparative Stopping Distances

Section Summary Questions

1. Whatisthefinalfactorthatwilldetermine

ifthevehiclewillmove?

2. Whatisthefinalfactorthatwilldetermine

ifthevehiclewillstop?

3. Howistheheatthatisgeneratedbythebrakesdissipated?

4. Ifonesetofbrakeshoesispoorlyadjusted,

whateffectcouldithaveontheremaining

setsofbrakeshoesinthesystem?

5. Whatismeantbythetermfriction?

6. Iftheweightofthevehicleisdoubled,howmany

timesmustthestoppingpowerbeincreased?

7. Ifthespeedofthevehicleisdoubled,howmany

timesmustthestoppingpowerbeincreased

tobeabletostopatthesamedistance?

8. Ifbothweightandspeedofthevehiclearedoubled,howmanytimesmustthestopping

powerbeincreasedtostopatthesamedistance?

9. Whatiscompressedair?

10. Whatdoestheabbreviationpsistandfor?

11.If40psiisexertedagainstadiaphragmof

30squareinchesinarea,whatarethetotal

poundsofforcethatcouldbeexerted?

12.Stoppingdistanceconsistsofwhatthreefactors?

13.Definethefollowingterms?

DriversReactionTime

BrakingDistance

BrakeLag


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Air Brake Manual

TheComponentsofanAirBrakeSystem

Section2


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12 Air Brake Manual

SectionOneofthismanualhasexplainedthatitis

possibletogainamechanicaladvantagethroughthe

useofleversandthatairunderpressurecanbeused

togainamechanicaladvantage.SectionTwowill

explainhowairunderpressurecanbeusedto

operatetheairbrakesofavehicle.

Pipingillustrationshavebeenkeptsimpleinordertobeeasilyunderstood.Thepipingarrangementsfound

onvehiclesinactualuseonthehighwaymightdiffer

somewhatfromtheillustrationsinthismanual.

The Components o anAir Brake System

Abasicairbrakesystemcapableofstopping

avehiclehasfivemaincomponents:

1.Acompressortopumpairwithagovernor

tocontrolit.

2.Areservoirortanktostorethecompressedair.

3.Afootvalvetoregulatetheflowofcompressedair

fromthereservoirwhenitisneededforbraking.

4.Brakechambersandslackadjusterstotransfer

theforceexertedbythecompressedairto

mechanicallinkages.

5.Brakeliningsanddrumsorrotorstocreate

thefrictionrequiredtostopthewheels.

Itisnecessarytounderstandhoweachofthese

componentsworkbeforestudyingtheirfunctions

intheairbrakesystem.

Compressor and Governor

Compressedairisusedtotransmitforceinanair

brakesystem.Thesourceofthecompressedairis

acompressor(1).Acompressorisdesignedtopump

airintoareservoirwhichresultsinpressurizedair.

Thecompressorisdrivenbythevehiclesengine,eitherbybeltsandpulleysorshaftsandgears.Invehicles

wherethecompressorisdrivenbybelts,theyshouldbe

checkedregularlyforcracksandtension.Also,checkthe

compressorforbrokenmountingbracketsorloosebolts.

Thecompressorisinconstantdrivewiththeengine.

Whenevertheengineisrunning,soisthecompressor.

Whenpressureinthesystemisadequate,anywhere

fromalowof80psitoahighof135psiitisnot

necessaryforthecompressortopumpair.Agovernor

(2)controlstheminimumandmaximumairpressure

inthesystembycontrollingwhenthecompressor

pumpsair.Thisisknownastheloadingor

unloadingstage.Mostcompressorshavetwo

cylinderssimilartoanenginescylinders.When

thesystempressurereachesitsmaximum,which

isbetween115and135psi,thegovernorplacesthe

compressorintheunloadingstage.

Thecompressormustbeabletobuildreservoirair

pressurefrom50to90psiwithinthreeminutes.

Ifunabletodosothecompressorrequiresservicing.

Acompressormaynotbeabletobuildairpressure

from50to90psiwithinthreeminutesiftheair

filterispluggedorifthebeltwasslipping,ifthese

werenotatfaultthecompressorcouldbefaulty.

Governor

Pressuresettingspring

Exhaust port

Unload port

Reservoir port

Exhaust port

Unload port

Reservoir port


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Air Brake Manual

Placingthecompressorintheunloadingstageisdone

bydirectingairpressuretotheinletvalvesofthe

compressor,holdingthemopen,allowingtheairto

bepumpedbackandforthbetweenthetwocylinders,

insteadofcompressingtheair.Whenthepressure

inthesystemdrops,theinletvalvesclose,returning

thecompressortotheloadingstage.Thegovernor

mustplacethecompressorintheloadingstageat

nolowerthan80psi.Duringtheunloadingstage,

thecompressorisabletocool.

Usuallycompressorsarelubricatedfromtheengine

lubricationsystem,althoughsomecompressorsare

selflubricatingandrequireregularchecksofthe

lubricantlevel.

Itisveryimportanttheairthatentersthesystem

bekeptascleanaspossible.Theairmustfirstpass

throughafiltertoremoveanydustparticles.Theair

filtermustbecleanedregularly.Adirtyfilterwill

restricttheflowofairintothecompressor,reducing

itsefficiency.Somevehicleshavetheinletportof

thecompressorconnectedtotheintakemanifold

andreceiveairthathasbeenfilteredbytheengine

aircleaner.

Apistontypecompressoroperatesonthesame

principleastheintakeandcompressionstrokes

ofanengine.

Intakestroke:Thedownwardstrokeofthepistoncreatesavacuumwithinthecylinderwhichcauses

theinletvalvetoopen.Thiscausesatmospheric

airtoflowpasttheinletvalveintothecylinder.

Piston

From governor

Intake air ilter

Unloadplunger

Inlet valve

Discharge valve

Compressor (Unloading stage)

Intake air ilter

Unloadplunger

Inlet valve

Discharge valve

Compressor (Intake stroke)

Piston


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14 Air Brake Manual

Compressionstroke:Theupwardmotionofthepistoncompressestheairinthecylinder.Therising

pressurecannotescapepasttheinletvalve(which

thecompressedairhasclosed).Asthepistonnears

thetopofthestroke,thepressurizedairisforced

pastthedischargevalveandintothedischargeline

leadingtothereservoir.

Reservoirs

Reservoirsortanksholdasupplyofcompressedair.

Thenumberandsizeofthereservoirsonavehicle

willdependonthenumberofbrakechambersand

theirsize,alongwiththeparkingbrakeconfiguration.

Airbrakevehiclesareequippedwithmorethanone

reservoir.Thisgivesthesystemalargervolumeofmain

reservoirair.Thefirstreservoirafterthecompressoris

referredtoasthesupplyorwet(5)reservoir.Theother

reservoirsareknownasprimary(8)andsecondary(10)

ordry(8)(10)reservoirs.Whenairiscompressed,

itbecomeshot.Theheatedaircoolsinthereservoir,

formingcondensation.Itisinthisreservoirthat

mostofthewateriscondensedfromtheincomingair.

Ifoilleakspastthepistonringsofthecompressor

andmixeswiththismoisture,itformssludge,which

accumulatesinthebottomofthereservoir.Ifallowed

toaccumulate,thissludge(waterandoil)wouldenter

thebrakingsystemandcouldcausetroublewithvalves

andotherparts.Inwinter,waterinthesystemmay

freeze,causingthemalfunctionofvalvesorbrake

chambers.Reservoirsareequippedwithdrainvalvesso

thatanymoistureorsludgethatmayhaveaccumulated

canbedrained.Ifyounoticesludgewhendrainingyour

system,haveitinspectedbyamechanic.Tominimize

theamountofwatercollection,allreservoirsmust

bedraineddaily.Underextremeconditions,reservoirs

mayhavetobedrainedmorethanonceaday.Todrain

thereservoirsalwaysstartwiththewetreservoiron

thetractor.Openthedrainvalvefullyandallowall

airpressuretoescape,whichwillalsoexhaustthe

moisturecollectedinthereservoir.

Somereservoirshavemorethanonecompartment

andeachcompartmenthasitsowndrainvalve,

whichmustbedrainedindividually.Brieflyopening

thevalvejusttoallowsomeoftheairtoescapedoesnotdrainthemoisture!Itisnotsafetoassumethat

thewetreservoir,orthepresenceofanairdryeris

reasontoneglecttheotherreservoirsonthepower

unit,trailersordollies.Theyshouldallbecompletely

draineddailybyopeningthedrainvalvefullyand

allowingalloftheairtoescape.Thisshouldbedoneat

theendoftheday.

Somereservoirsmaybeequippedwithautomatic

reservoirdrainvalves(spittervalves).Thesevalves

willautomaticallyexhaustmoisturefromthe

reservoirwhenrequired,althoughtheyshould

becheckeddailyanddrainedperiodicallytoensurethemechanismisfunctioningproperly.Anyloose

ordisconnectedwiresassociatedwiththevalve

heatersshouldberepairedimmediately.

Reservoir

Intake air ilter

Unloadplunger

To reservoir

Inlet valve

Discharge valve

Compressor (Compression stroke)

Piston


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Air Brake Manual

Air Dryer

Anairdryer(3)maybeinstalledbetweenthe

compressorandthewetreservoirtohelpremove

moisturefromthecompressedair.Itmaybepartially

filledwithahighmoistureabsorbentdesiccantand

anoilfilter,oritmaybehollowwithbafflesdesigned

toassistinseparatingthemoisturefromtheair.Both

typesofairdryersuseairpressuretopurgeorejectthe

accumulatedcontaminantsfromtheirdesiccantbed.

Thepurgevalvehasaheaterelement,whichprevents

themoisturefromfreezingincoldclimateoperation.

Thewiringconnectedtotheheatershouldbeinspecte

forlooseordisconnectedwires.Theyarealsoequipped

withasafetyvalve.

Controlport

Supplyport

Cutopiston

Exhaust

Purge

valve

Delivery port

Oneway check valve

Oneway check valve

Oriice

Desiccant bed

Control PortDried Ai

Check valvassembly

Delivery Port

Heaterelement

ExhaustPurgevalve

Cutopiston

SupplyPort

Reservoir

Compressor

GovernorSump

Air Dryer (Purge cycle)

DesiccaCartridg

Air Dryer (Drying cycle)

Air Dryer

Control Port

Supply Port

Oil Separator


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16 Air Brake Manual

Saety Valve

Asafetyvalve(4)protectsreservoirsfrombecoming

overpressurizedandburstingifthegovernor

malfunctionedanddidnotplacethecompressor

intheunloadingstage.Thevalveconsistsofa

springloadedballthatwillallowairtoexhaust

fromthereservoirintotheatmosphere.Thevalves

pressuresettingisdeterminedbytheforceofthe

spring.Asafetyvalveisnormallysetat150psi.

Ifthepressureinthesystemrisestoapproximately

150psi,thepressurewouldforcetheballoffitsseat,

allowingthepressuretoexhaustthroughtheexhaust

portinthespringcage.Whenreservoirpressure

issufficientlyreducedtoapproximately135psi,

thespringwillforcetheballbackontoitsseat,

sealingoffthereservoirpressure.Notallsafety

valveshaveamanualreleasefeature.

Ifthesafetyvalvehastorelievepressure,thegovernor

orcompressorrequiresadjustment,serviceorrepair.

Thisshouldbedonebyaqualifiedmechanic.

Foot Valve

Thefootoperatedvalve(31)isthemeansofapplying

airtooperatethebrakes.Thedistancethetreadle

ofthefootvalveisdepressedbythedriverdetermines

theairpressurethatwillbeapplied,butthe

maximumapplicationwillnotexceedthepressure

inthereservoir.Releasingthefootvalvetreadle

releasesthebrakes.

Whenthedriverappliesthebrakes,depressingthe

treadlepartway,thefootvalvewillautomatically

maintaintheapplicationairpressurewithoutthe

driverhavingtoadjustthepressureofhisfooton

thetreadle.

Releasingthetreadleallowstheapplicationairto

bereleasedthroughtheexhaustportsintothe

atmosphere.Airtreadlesarespringloaded,producing

adifferentfeelfromhydraulicbrakeapplications.

Brake Chambers, Slack Adjustersand Brake Lining

Brake Chamber and Slack Adjuster(Brakes o)

Treadle

Treadlespring

Supply romreservoir

Exhaust Port

Push rod

Brake chamber

DiaphragmDiaphragm

return spring

Air inlet

Mounting bolts

Clevis and pin

Slackadjuster

Air pressure greater than 150 psi

Saety Valve

Foot Valve

To brakechambers


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Air Brake Manual

Abrakechamber(11)(14)(32)isacircularcontainer

dividedinthemiddlebyaflexiblediaphragm.

Airpressurepushingagainstthediaphragmcauses

ittomoveawayfromthepressure,forcingthepush

rodoutwardagainsttheslackadjuster.Theforce

exertedbythismotiondependsonairpressureand

diaphragmsize.Ifaleakoccursinthediaphragm,

airisallowedtoescape,reducingtheeffectiveness

ofthebrakechamber.Ifthediaphragmiscompletely

ruptured,brakesbecomeineffective.

Frontbrakechambers(32)areusuallysmallerthan

thoseintherearbecausefrontaxlescarrylessweight.

Abrakechamberisusuallymountedontheaxle,

nearthewheelthatistobeequippedforbraking.

Airpressureisfedthroughaninletport.Theair

pushesagainstthediaphragmandthepushrod.

Thepushrodisconnectedbyaclevisandpinto

acrankarmtypelevercalledaslackadjuster.

Thisconvertsthepushingmotionofthepushrod

fromthebrakechambertoatwistingmotionofthe

brakecamshaftandScams.Whentheairisexhauste

thereturnspringinthebrakechamberreturnsthe

diaphragmandpushrodtothereleasedposition.

Asindicatedbyitsname,theslackadjusteradjusts

theslackorfreeplayinthelinkagebetweenthe

pushrodandthebrakeshoes.Thisslackoccurs

asthebrakeliningswear.Iftheslackadjusters

arenotadjustedwithinthelimitations,effective

brakingisreducedandbrakelagtimeisincreased.

Iftoomuchslackdevelops,thediaphragmwilleventuallybottominthebrakechamber,and

thebrakeswillnotbeeffective.

Illustratedbelowaretwocommontypesofmanual

slackadjusters,showingthewormadjustinggear.

Push rod

Brake chamber

Diaphragm Diaphragmreturn spring

Air inlet

Mounting bolts

Clevis and pin

Slackadjuster

Manual Slack Adjusters

Ball Indent Slack Adjuster Positive Lock Slack Adjuster

Lock screw

Adjusting boltWorm shat

Worm gear

Locking collar

Spline Spline

Grease itting

Adjusting bol

Brake Chamber and Slack Adjuster (Brakes on)


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18 Air Brake Manual

Onmanualslackadjusters,theadjustingwormbolt

isturneduntilthebrakeliningstouchthedrumsandthenbackedoff,normallyNtoMaturn.Alocking

device,whichmaybeaspringloadedcollaroverthe

headoftheadjustingbolt,mustbedepressedwhen

thewrenchisslippedoverthebolthead.Thisis

knownasapositivelockslackadjuster.

Ortheycoulduseaspringloadedinternalcheckballto

locktheadjustment,anditmustberemovedtomake

anyadjustment.Thisisknownasaballindentslack

adjuster.Themoreoftenthedrivercheckstheslack,

thelesstheprobabilityofbrakefailure.Vehiclesrarely

losetheirbrakesbecauseofairloss;itisusually

becausetheyareoutofadjustment.

Itisthedriversresponsibilitytoensurethatbrakesare

adjustedcorrectly.Asimpleservicebrakeapplication

atlowspeedtocheckbrakeadjustmentisnotadequate.

Brakingathighwayspeedcausesbrakedrumexpansion

duetoheat,whichinturnrequiresgreaterpushrod

traveltomaintainthesamebrakingforce.Ifabrake

isoutofadjustmenttherewouldnotbeenoughreserve

strokeofthepushrodtraveltocompensatefordrum

expansion.Thiswouldcauseabrakefadeandwould

greatlyextendstoppingdistance.Iftravellingdown

ahill,thiscouldcausecompletebrakeloss.

Note:Detailedbrakeadjustmentproceduresare

outlinedinSection8.

Pushrod

Airinlet

Slackadjuster

Brake Chamber and Slack Adjuster (Brakes on)

Thrust washer

Clevis

Clevis pin (large)Clevis pin (small)

Actuator rod

Hairpin clip

Boot and strap

Actuator (adjusting sleeve)

Roller (pin)

Actuator piston

Pressure relie capscrew (pull pawl)

Pawl spring

Adjusting pawl

Worm

Worm seal

Adjusting bolt

Grease groove

Grease itting

Housing

Worm gear

Automatic Slack Adjuster


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Somesystemshaveautomaticslackadjustersthat

adjustautomaticallytocompensateforbrakelining

wear,usuallymaintainingthecorrectclearance

betweenthebrakelininganddrum.Automaticslack

adjustersmustbecheckedregularlytoensurethat

correctadjustmentisbeingmaintained.Thereare

variousmakesandmodelsofautomaticslackadjusters

inuse.Primarily,theyareeitherstrokesensingor

clearancesensing.Astrokesensingadjusterwilladjust

theslackwhenitsensesthesetstrokeisexceeded.

Aclearancesensingadjusterwilladjustwhenthe

properclearancebetweenthebrakedrumandbrake

shoeisnotmaintained.Someautomaticslackadjusters

havetheabilitytobackofforincreasetheslackwhen

ithasoveradjustedthebrake.Ifavehicle

isequippedwithautomaticslackadjusters,itshould

notbetakenforgrantedthatthebrakeswillalways

beinadjustment.Thesystemisnotfoolproof.

Anumberoffactorscouldresultintheautomatic

slackadjusternotmaintainingproperslack.Therecouldbeimproperinstallation,inadequate

maintenance,deformedbrackets,worncambushings,

bentpushrods.Evenpoorvisualinspectioncanresult

inproblemsunrelatedtoadjusterfunction.When

conductingapretriporposttripairbrakeinspection,

lookforwornordamagedcomponents.Automaticslack

adjusterscanmalfunctionandnotkeepthebrakein

adjustment,especiallywhenithasbeeninservice

foralongperiodoftime.Thetwomostcommon

problemsareexcessiveprematurewearandinternal

contamination.Asanautomaticslackadjusterages

inservice,thecomponentswearthatsensewhenan

adjustmentisrequired.Theresultismorestrokeisrequiredfortheliningtocontactthebrakedrum,and

ifnotcheckedthebrakecouldbeoutofadjustment.

Ifevenasmallamountofwaterissuckedintoan

automaticslackadjustermechanismitcancause

corrosionor,inwinter,itcanfreezetheinternal

sensingcomponentsandinhibitorpreventadjustmen

Also,undercertainconditions,anautomaticslack

adjusterthatdoesnothavetheabilitytobackoff

orincreaseslack,mayoveradjustabrakecausing

ittodrag.Forexamplethiscouldtakeplacewhena

tractortrailerisnegotiatingalong,curvingdowngrad

Thedrivershouldsnubthebrakes,whichis

repeatedlyapplyingthebrakesmoderatelytomaintain

safecontrolofthevehicle.Howeveritwouldnottake

longinthisseverebrakingconditionforoneormore

ofthebrakedrumstooverheatandexpand.Theover

heatingwillphysicallyincreasethebrakedrums

diameter,andinextremeandprolongedconditions

willleadtolongerpushrodstrokestoachievethe

brakingforcerequired.Theautomaticslackadjuster

interpretsthisasaneedforadjustmentandwill

takeupslack.Whenthebrakedrumcoolsdownand

returnstonormalsizethebrakesareoveradjusted

anddragging.Atthattimethedrivershouldstopandcheckthebrakesforadjustment.Anumber

offullbrakeapplicationsabove90psiperdayare

requiredtokeeptheautomaticslackadjustersin

adjustment(seeSection8formoreinformation).

Becauseautomaticslackadjustersarenotfoolproof,

itisimportanttheoperatorofavehicleequipped

withautomaticslackadjustersbeabletomanually

adjustthem.Forinformationonmanuallyadjusting

theautomaticslackadjustersonyourvehicleconsult

themanufacturer.

Illustratedisacommontypeofbrakeassemblyused

ontruckrearaxlesandtraileraxles.Afrontaxleassemblyhasthebrakechamberandslackadjuster

mountedonthebackingplatebecauseofthe

steeringaction.

Brake drum

Brake chamber

Push rod, clevis and pin

Slack adjuster

Scam

Brake lining

Brake Assembly


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20 Air Brake Manual

Brakeliningmaterialisattachedtotheshoes.The

materialuseddependsonthebrakingrequirementsof

thevehicle.Brakeliningmustgiveuniformoutputof

brakeeffortwithminimumfadeathightemperatures.

Fadingorreductioninbrakingeffortoccurswhenthe

heateddrumsexpandawayfromthebrakelinings.

Thebrakeliningsalsolosetheireffectivenesswithoverheating.

ThetwistingactionofthebrakecamshaftandScam

forcesthebrakeshoesandliningsagainstthedrums.

Thebrakeliningsgenerateheatfromfrictionwith

thebrakedrumsurface.

Thethicknessofthedrumsdeterminestheamount

ofheattheyareabletoabsorbanddissipateinto

theatmosphere.Drumswornthinwillbuildup

heattooquickly.Dangerouslyundependablebrake

performancewillresultfromdistorteddrums,

weakreturnsprings,improperlining,poor

adjustment,orgreaseordirtonthelining.Drumsmustneverbemachinedorwornbeyond

themanufacturersspecification.

Wedge Brakes

Thisisanotherexampleofabrakeassemblyusedon

someairbrakeequippedvehicles.Theactionofthe

brakechamberpushrodforcesawedgeshapedpush

rodbetweenthebrakeshoerollers.Thisforcesthe

brakeshoeliningagainstthebrakedrum.

Thevehiclemaybeequippedwithasingleordual

chambersoneachwheel,dependingonthevehicles

sizeandstyle.

Thesebrakesmaybeequippedwithaselfadjusting

mechanismorwithamanualstarwheeladjuster.

Thestarwheeladjustmentismadewiththevehicle

jackedup,toinsurethatthebrakeliningsdonotdrag.Manualadjustmentofwedgebrakesisusuallydone

byaqualifiedmechanic.

Brakechamber

Adjusting wheel

Brake shoeroller

Push rod

Shoe returnspring

Brake lining

Brake shoe

Wedge Brake Single Chamber

Brakechamber

Single chamber Dual chamber

Shoe returnsprings

Brake lining

Adjusting wheel

Brakechambers

Adjusting wheel

Wedge Brakes


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Air Brake Manual

Disc Brakes

Theairactivatedheavytruckdiscbrakeissimilar

inprincipletothatusedonpassengervehicles.

Airpressureactsonabrakechamberandslack

adjuster,activatingthebrakes.Insteadofthecam

orwedgeusedinconventionalheavytruckdrum

brakes,apowerscrewisused.Apowerscrewworks

likeaCclamp,sothattheliningpadsexertequal

forcetobothsidesofthediscorrotor.Sometypes

ofdiscbrakeshaveabuiltinautomaticadjuster.

Discbrakesthatrequiremanualadjustmenthave

adjustmentspecificationsthatdifferfrom

conventionalScambrakingsystems.Alwayscheck

themanufacturersspecificationsbeforeadjusting.

Discbrakeassembliesmayhaveaspringparking

brakeunitattachedtotheservicebrakechamber.

AirOverHydraulic Brake Systems

Airoverhydraulicbrakesystemsweredeveloped

formediumweightvehiclesbecause:

dieselenginesdonothaveasourcefor

vacuumboostingunlesstheyareequipped

withavacuumpump. mediumweightvehiclesdonotrequire

afullairbrakesystem.

itgivestheoptionofpullinganairbrake

equippedtrailer.

Thesesystemscombinethebestfeaturesof

anairandhydraulicbrakesystem.Theyuse

hydraulicbrakesateachwheelwiththeir

reliableselfadjustersandlimitedmaintenance.

Onthesesystemstheairisusedtoeitheractuate

thehydraulicbrakesorboostthehydraulic

brakepressureasexplainedinthefollowing.

AirActuated Hydraulic Brake System

(Air Brake Endorsement Required)

Anairactuatedsystemusuallyhasthesame

componentsofastandardairsupplysystemincluding

awarningbuzzerandlight,compressor,governor,

wetanddryreservoirs,andafootvalvethatcouldbe

asingleordualtype.Thesecomponentsarefound

usuallyinthesameplacesasonafullairbrake

system.Alsothereareoneortwoairactuated

hydraulicpressureconvertersdependingonifthe

systemisasingleoradualsystem.Thissystem

consistsofanairchamberorcylinderattachedto

ahydraulicmastercylinder.Whenthefootvalve

isdepressed,theairpressureactuatesthepushrod

fromtheairunitthatpushesagainstthemaster

cylinderpiston,producinghydraulicpressure

directedthroughtubingtothewheelcylinders

actuatingthefrontandrearaxleservicebrakes.

Disc Brake


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22 Air Brake Manual

Itisessentialthattheoperatorofsuchavehiclehave

knowledgeofairpressurebuilduptime,governor

loadingandunloadingpressure,warningdevice

operation,andhowtodrainairreservoirsproperly

(seeSectionNine;PreTripAirBrakeInspection).

Ifanairactuatedhydraulicbrakesystemwastolose

itsairsupply,thevehiclewouldhavenoservicebrakes.

Onlytheparkingbrakewouldbeoperatingasitismechanicalandrequiresnoairpressuretooperate.

Eachvehiclemanufacturermayhavedifferentparking

brakeapplications,eitherautomaticallywhenair

pressureisreducedinthereservoir,ormechanically

byabrakeontherearofthetransmission,orwith

therearbrakesystem.Sincehydraulicbrakesystems

actuatedbyairpressureareregardedasanairbrake

system,yourdriverslicencemusthaveanairbrake

endorsementforyoutooperatevehiclesequipped

withairactivatedhydraulicbrakes.

Astherearemanydifferentsystemsinuse,referto

theoperatorsmanual.

AirBoost Hydraulic Brake System

(Air Brake Endorsement not Required)

Anairboosthydraulicbrakesystemusesairpressure

toassistbrakeforce.Thisissimilartovacuumassisted

brakesonmostpassengervehicles.Anairboostsystem

usuallyhasthesamecomponentsofastandardair

supplysystemincludingacompressor,governor,wet

anddryreservoirs.Thesecomponentsarefoundusually

inthesameplacesasonafullairbrakesystem.

Thebrakepedallinkageoperatesahydraulicmaster

cylinderthatsendshydraulicpressuretothebooster

unit.Initially,atlowpressurethehydraulicfluid

passesthroughtheboosterandbeginstopressurize

thewheelcylindersmovingthebrakeshoesouttothe

drums.Theseboosterunitsaresimilarinoperationto

HypowerorHydrovacvacuumboostersfound

onmostlightandmediumweightvehicles,butair

pressureisusedtointensifythehydraulicpressure

generatedbythemastercylinderratherthanvacuum.

Builtintotheboosterunitisahydraulicallyoperatedaircontrolvalve.

Air linesReservoirsCompressor

Foot valve

Hydraulic lines

Air brake chamber

Hydraulicwheel

cylinders

Hydraulicwheel

cylinders

Air lines

Air brakechamber

Hydraulic mastercylinder

Hydraulic mastercylinder

AirActuated Hydraulic Brake System


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Thisiswhereairfromthereservoirisdirected.

Asthepressurefromthemastercylinderincreases,

theaircontrolsectionintheboosterwillopen

andbegintodeliverairpressuretotherearofthe

aircylinder.Theaircylinderpushrodtransfers

pressureonapistoninthehydraulicsectionofthe

booster,increasingthehydraulicpressureatthe

wheelcylinders.

Thedriverhasfullcontrolofthebrakingforceas

theaircontrolsectionmodulatestheboostpressure

inproportiontothemastercylinderpressure.

Ifthevehiclewastolosealloftheairpressure

thebrakesystemwouldlosetheairassistboost,

howeverthehydraulicsystemwouldcontinueto

workbutatreducedeffectiveness.Anairbrake

endorsementonadriverslicenceisnotrequired

tooperateavehiclewiththisbrakesystem.Consult

theoperatorsmanualforthevehicleyoudrivefor

maintenancerequirements.

Air lines

Hydraulicwheel

cylinders

Compressor

Reservoir Hydraulic mastercylinder

Brakepedal

Hydraulic lineBooster unit

AirBoost Hydraulic Brake System

Air lines Booster unit

Hydraulicline

Hydrauwheel

cylinde


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24 Air Brake Manual


1. Whatarethefivebasiccomponentsofan

airbrakesystem?

2. Atwhatpressureshouldthegovernorcausethe

compressortoreturntoitsloadingstage?

3. Atwhatpressurewillthegovernorplacethecompressorintheunloadingstage?

4. Howisapluggedairfilterlikelytoaffectthe

aircompressor?

5. Whatcausesmoisturetoformintheair

brakesystem?

6. Whenisthecompressorabletoaccomplish

mostofitscooling?

7. Howaremostcompressorslubricated?

8. Howoftenshouldthereservoirsbedrained?

9. Isitnecessarytoallowallthepressuretoescape

fromthereservoirinordertoremovethemoisture

andsludgewhichmayhaveaccumulated?

10. Whatisthemaximumpressureavailablefor

afullbrakeapplicationatanygiventime?

11. Whatwillresultifthebrakedrumsareworn

thinorturnedtoofar?

12. Ifthegovernorvalvefailedtounloadthe

compressor,whatwouldprotectthereservoirs

frombecomingoverpressurizedandbursting?

13. Whatisthepurposeofhavingmorethan

onereservoir?

14. Whataretwofunctionsoftheslackadjusters?

15. Doestheamountofslackinthebrakelinkages

haveanyeffectonthebrakingefficiencyof

thevehicle?

16. Whatistheadvantageofkeepingthebrake

chamberpushrodtraveladjustedwithin

limitations?

17. Whatisthemostcommoncauseoflossof

effectivebrakinginanairbrakesystem?

18. DoautomaticslackadjustersonScambrakes

requirechecking?

19. Cantheadjustmentonairoperateddisc

brakesdifferfromScambrakes?

20. Whatoccurswhendrumbrakes

becomeoverheated?

21. Whatcausesbrakefade?

22. Whatisthemainfunctionofthefootvalve?

23. Whydoesthefeelofanairoperatedfoot

valvedifferfromahydraulicbrakepedal?

24. Onwhatprincipledoesadiscbrakeoperate?

25. Whattypeofairoverhydraulicbrakesystem

requirestheoperatortoholdanairbrake

endorsement?


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HowtheBasicSystemWorks

Section3


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26 Air Brake Manual

Basic Air Brake System

Airispumpedbythecompressor(1)tothewetreservoir

(5),whichisprotectedfromoverpressurizationbya

safetyvalve(4).Thegovernor(2)controlsthepressure

inthereservoirtothebottomofthefootvalve(31).

Thedriverpushesthefootvalvetreadledownandair

pressureflowstothefrontandrearbrakechambers

(32&11).Thebrakechamberpushrodsmovetheslack

adjusters.TheslackadjustersrotatetheScams,forcing

thebrakeshoesagainstthedrums.Thiscausesfriction

thatstopsthewheels.Thedriverreleasesthefootvalve

treadleandtheairinthebrakechambersisallowedto

exhaustthroughthefootvalve,releasingthebrakes.

Thefollowingexplainstheadditionalcomponents

ofabasicairbrakesystem.Othervalveswhichare

necessarytoensuresmoothandefficientoperations

arenotincludedinthissimpledrawing.Theywill

bediscussedlaterinthemanual.

Note:Anairdryer(3)hasbeenaddedtoreduce

theamountofmoistureinthesystem.

OneWay Check Valve

Inthediagrambelow,tworeservoirsareshown(5)(10).

Topreventairfromflowingbackwardsinthesystem

towardthecompressor,aonewaycheckvalve(7)

isinstalledbetweenthereservoirs.Thisvalveallows

theairtoflowinonedirectiononly.Thevalveis

springloaded.

Pressureattheinletsideovercomesthespring

pressureandliftsthecheckvalveball,ordisc,off

itsseat.Airpassesthroughthevalvetotheoutlet.

Whenpressureattheoutletbecomesgreaterthan

attheinlettogetherwiththespringpressurethe

checkdeviceseats,preventingairfromflowingback

throughthevalve.

1

2

3 4 5 7 9 1032 11

32 31 11

Basic Air Brake System

Ball

Spring

Body

Cap nut

OneWay Check Valve


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Air Brake Manual

Air Pressure Gauge

Vehicleswithanairbrakesystemareequipped

withareservoirairpressuregauge(29).Thisgauge

ismountedinthecab,usuallyonthedashboardand

indicatestheairpressureintheprimaryandsecondary

ordryreservoirs.Thesupplyorwetreservoirdoesnot

usuallyhaveanairpressuregauge.Commonoperating

pressuresare80to135psi,dependingonthesystem.

Monitoringthegaugewillalertthedrivertoany

unusualchangesinairpressure.

Brake Application Gauge

Anadditionalgaugecanbeinstalledonthedashto

indicatetheapplicationairpressurewhenthebrakes

areapplied.Thisgaugecanbepipedtoindicatethe

pressureofeitherafootorhandapplication.(Hand

applicationwillbeexplainedlaterinthemanual.)

Low Pressure Warning Device

Allvehiclesequippedwithanairbrakesystemmust

haveadevicetowarnthedriveriftheairpressurein

thesystemdropstoadangerouslevel.Thisdevicecoul

bearedwarninglight,abuzzerorawigwag.Dueto

overuseorleaks,thelowpressureindicatorswitch(9)

willturnonaredwarninglightonthedashorcause

abuzzertosoundatorbefore60psi.Somevehicles

areequippedwithbothalightandabuzzertowarn

thedriverofalowairpressurecondition.

Wigwagsarenotfoundinmodernvehicleshaving

beenreplacedwitharedwarninglightandbuzzer.

Theymaystillbeinuseonoldervehicles.Thereare

twotypesofwigwaglowpressurewarningdevices

thatmaybeused.Bothtypeswilldropintothedriver

viewshouldthesystempressuredropto60psi.The

automaticwarningdevicewillriseoutofthedrivers

viewwhenthepressureinthesystemrisesabove60

psi.Themanualresettypemustbeplacedintheoutofviewpositionmanuallyandwillnotstayinplace

untilthepressureinthesystemgoesabove60psi.

Whicheverwarningsystemisused,buzzerlights

orwigwag,thedrivermuststopthevehiclein

asafeplaceimmediatelyandfindthecauseof

theairloss.Theairpressureremaininginthe

system(approximately60psi)isenoughforabrake

applicationifthedriveractspromptly.

Keepinmindifthevehiclehasspringparking

brakes,at60psithespringparkingbrakesare

partiallyappliedandyoucannotreleasethemuntil

airpressureincreases.Ifyoudonotmoveyourvehicletoasafeplaceyoumayfindyourselfstranded

inthetraveledportionofthehighway.

Stop Light Switch

Anydriverfollowingyourvehiclemustbewarned

whenreducingspeedorstoppingthevehicle.Thestop

lightswitch(25)isanairoperatedelectricswitchthat

turnsonthebrakelightsattherearofthevehicle

whenabrakeapplicationisbeingmade.


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28 Air Brake Manual

Quick Release Valve

Theapplicationofthebrakesinthebasicsystemwas

describedearlier.Inabasicsystem,whenthedriver

releasesthefootvalve,itwouldbenecessaryforthe

airunderpressureinthebrakechamberstoreturn

tothefootvalvetoreleasethebrakes.Thisreleasing

actionwouldbeslowedinlongwheelbasevehicles

becauseofthelongerlinesbetweenthefootvalve

andtherearbrakechambers.Aquickreleasevalve

(33)isinstalledtoallowthebrakestoreleasequickly

andfullybydischargingtheapplicationairnearthe

brakechambers.Toallowthebrakestoreleasequickly

andfullybydischargingtheapplicationairnearthe

brakechambers.

Relay Valve

Thefootvalveisusuallylocatedclosertothefront

wheelsthantotherearwheels.Thelongerthedistance

fromthefootvalvetotherearchambers,themoretime

itwilltakebeforetherearbrakesapply.Thisisknown

asbrakelag.Tocorrectthisconditiononalongwheel

basevehicle,arelayvalve(13)isinstalledneartherear

brakechambers.Alargerdiameterpipeisconnected

betweenthemainreservoirandtherelayvalve.The

airlinefromthefootvalvetotherelayvalvenow

becomesacontrolline.(Theairinthecontrolline

deadendsattherelayvalve.)Whenthefootvalveis

depressed,theairpressureinthecontrollineacts

onthetopsectionoftherelayvalve,relayingreservoir

airdirectlytotherearbrakechambersthroughthe

largerdiameterpipe.Thepressureofthereservoirair

deliveredinthiswaywillbethesameasthecontrol

pressuredeliveredbythefootvalve.

Releasingthefootvalveexhauststhecontrolairtothe

relayvalve,allowingittocutofftheflowofreservoir

airtotherearchambers.Thisinturnexhauststheair

inthebrakechambersbythequickreleasefeatureof

therelayvalve.

Manual Front Brake Limiting Valve

Forbettersteeringcontrolwhenbrakingonaslippery

roadsurface,itcanbeanadvantagetoreducethe

brakingefforttothefrontwheels.Thiscanbe

accomplishedbyinstallingacontrolvalve(35)in

thecab,andafrontbrakelimitingvalve(36)on

thefrontaxle.

Thecontrolvalveissetinthenormalpositionfor

dryroadsurfacesandthefrontbrakingapplication

airpressureisnormal.Onaslipperyroadsurface,

thecontrolvalve(35)issettotheslipperyroadposition.Inthisposition,thecontrolvalvewill

causethelimitingvalve(36)tooperate.Applying

airpressuretothefrontbrakesisthenreduced

to50percentoftheapplicationairpressure

beingdeliveredtotherearbrakechambers.

Delivery ports not shown

Manual Front BrakeLimiting Valve

Dash MountedControl Valve

Service port Lever

Exhaust port Service port

To limitingvalve

Fromlimiting

valve

Relay Valve

Quick Release Valve


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Air Brake Manual

Somesystemsareequippedwithanautomaticlimiting

valve(34).Thisvalvewillholdoffbrakeapplication

tothefrontwheelsfrom0to10psi,dependingon

howithasbeenpreset.Betweenthepresetpressure

and40psiofbrakeapplication,thereductionis

approximately50percent.Brakeapplicationsbetween

40psiand60psiarereducedbylessthan50percent.

Brakeapplicationsmorethan60psiarenotreduced

andfullapplicationisdirectedtothefrontwheels.

1

2

3 4

5

6 7 6 8 932 11

1135

31

32

29

1336

Service port Service port

Delivery port Delivery port

Piston spring

Inletexhausvalve springLower piston

assembly

Automatic Front Brake Limiting Valve

Basic Air Brake System with Manual Front Brake Limiting Valve


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30 Air Brake Manual

Theairbrakesystemdiscussedpreviouslyisfor

avehiclewithasinglerearaxle.Thediagram

illustratesanairbrakesystemforavehicle

equippedwithanautomaticfrontbrakelimiting

valve(34),aquickreleasevalve(33)andatandem

setofrearaxles.Bothaxlesofthetandemsetare

equippedwithbrakes.

Arelayvalve(13)hastwouses:toprovide

aquickerapplicationofairpressuretothetandemrearaxlebrakeswhenabrakeapplicationismade,

andtoreleasethebrakesquickerwhenabrake

applicationisreleased.


1. Howcanthedrivertellhowmuchairpressure

isinthemainreservoirs?

2. Whatmustthedriverdowhenalowpressure

warningsystemactivates?

3. Whatisthepurposeofaquickreleasevalve?

4. Whatisthepurposeofarelayvalve?5. Whatisthepurposeofusingalargerdiameter

pipebetweenthereservoirandtherelayvalve?

6. Ifthefrontbrakelimitingvalveisinthe

slipperyroadposition,andthefootvalve

isdepressedtomakeabrakeapplicationof

30psi,howmuchpressurewillbeapplied

inthefrontbrakechambers?

7. Howisthereservoirprotectedfrom

overpressurization?

8. Whatstopspressurizedairfromflowingfrom

thedryreservoirbackintothecompressor?

9. Atwhatpressureshouldthelowpressure

warningdeviceactivate?

10. Howisbrakelagtotherearwheelsminimized?

11. Whenshouldadriverusethefrontbrake

limitingvalve?

34

33 13

Tandem Rear Axles


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Air Brake Manual

SpringParkingBrakes

Section4


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Spring Parking Brake Systems

(Single circuit system only)

Theinstallationofspringparkingbrakesandtheir

pipingarrangementsintoavehicleairbrakesystem

willvarydependingonthevehiclemake.

Springparkingbrakesmaybeinstalledonanairbrakeequippedvehicleforuseasareliableparking

brakesystem.Intheservicebrakesystem,thebrakes

areappliedbyairpressureandretractedbysprings.

Inthespringparkingbrakesystem,thebrakesare

appliedbyspringpressureandretractedbyairpressure.

Thespringparkingbrakechambersareattachedtothe

servicebrakechambersandoperatethroughthesame

linkage,thereforetheeffectivenessofthespring

parkingbrakedependsontheservicebrakeadjustment.

Thespringparkingbrakecontrolvalve(operatedby

asquare,yellowbutton)locatedinthecaballowsthe

drivertoexhaustairoutofthespringparkingbrake

circuittoapplythebrakes,orpressurizethecircuittoreleasethem.Somesystemsmayhaveanadditional

valvecontrolledbyabluebuttonthatappliesonly

thetractorspringparkingbrakesandnotthetrailer

springparkingbrakes.Thesystemcanalsoactas

anemergencybrake.Lossofairfromthesystem

mayautomaticallyapplythebrakes.

Controlvalveswillvary,dependingonthe

manufacturerandtypeofpipingarrangements.

Aspringloadedvalverequiresthatthevalvebe

pushedintoreleasethespringparkingbrakes.

Thisvalvecannotbeleftinthereleasedposition

withairpressureinthesystembelowapproximately

35psi.Anytimethereservoirpressuredropsto

approximately35psi,thisvalvewillclose

automatically,placingthespringparkingbrakesinto

fullapplication.Onsomeoldervehiclestheremaybe

asingletypeofpushpullcontrolvalvethatdoesnot

haveanautomaticreleasefeature.Toapplythespring

parkingbrakes,thevalvemustbeoperatedmanually,

eventhoughthereservoirpressurehasbeendepleted.

Duringnormaloperation,airpressurecages

(compresses)thespring,holdingitreadyfor

parkingoremergencybraking.

32 Air Brake Manual

Mounting Bolts

Springparkingbrake

chamber

Service brakechamber

Clevis and pin

Push rod

Diaphragm

Dustcap

Brakes O

Parking brakespring

Slackadjuster

Diaphragmreturnspring


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Air Brake Manual

Onthepretripairbrakeinspection(Section9),

youmustensurethattheparkingbrakespringis

notmanuallycagedoritwillnotexpandandapply

thebrake.Thebrakechambersshouldbechecked

forcracksanddamage.Thebrakechambershould

befittedwithadustcaptoensuredebriswillnot

enterthechamber.

Duringnormalservicebrakeoperation,

theparkingbrakespringdoesnotexpand.

Airpressurekeepsthespringcaged.

However,ittakesapproximately90psi

ofairpressuretokeepthespringscaged.

Ifreservoirairpressureisallowedtodrop

below90psithespringswillstarttomove.

Atapproximately70psithebrakeswillbe

partiallyappliedmakingitdifficultto

continuedriving.Iftheairpressuredrops

below60psithebrakeswillbefullyapplied

makingitimpossibletomovethevehicle

untilairpressureisbuiltuptorecagethe

springparkingbrakesorthespringsare

manuallycaged.

Springparkingbrakes(12),addedtothebrake

chambersoftherearaxleonthesingleunitvehicle,

areillustrated.Acontrolvalve(27)ismountedinthe

cab.Asupplylineofreservoirairispipedfromthedry

reservoirtothecontrolvalve.Openingthecontrol

valveallowsreservoirairpressuretoflowtothe

springparkingbrakechambers,releasingthem.

Service Brakes AppliedBrake On

12

1227

Using a Spring Parking Brake

Mounting Bolts

Clevis and pin

Slackadjuster

Pushrod

Diaphragm returnspring

Diaphragm

Springparking

brakechambe

Dustcap

Parking brakespring



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34 Air Brake Manual

Closingthecontrolvalveshutsoffthesupply

ofreservoirairpressureandexhauststhe

existingpressureinthespringparkingbrake

chambers.Thismotionallowsthespringto

expand,applyingthebrakes.

Caution:Parkingbrakesshouldbeintherelease

positionbeforemakingaservicebrakeapplication.

Afullbrakeapplication,madewhentheparking

brakesareapplied,cancompoundtheforceexerted

ontheslackadjustersandlinkageandresultin

damageorbrakefailure.Compoundingisthe

combinationoftwoforces:theforceappliedbythespringbrakechamberandtheservicebrakechamber.

Springbrakesareprimarilyusedasaparkingbrake,

butintheeventoflossofairpressureinthesystem,

theycanassistinstoppingthevehicle.Howquickly

theywillstopthevehicledependsonsuchfactorsas:

theweightandspeedofthevehicle;

thesteepnessofthegrade;

thespringforceofthespringbrakesthat

havebeeninstalled;and,

theadjustmentoftheservicebrakes.

Ifthebrakeshaveoverheated,suchasduring

mountaindrivingorhardhighwaybraking,care

mustbetakenwhenparkingthevehicle.Ifthe

springparkingbrakesareappliedwhenthebrake

drumhasexpandedbecauseofextremeheating,

whenthebrakedrumstartstocoolandcontract,

thepressureexertedbythespringparkingbrakemaycausethebrakedrumtocrackorwarp.When

parkingavehiclewithoverheatedbrakes,park

onlevelground,stoptheengineandleavethe

transmissioninthelowestgearandblockthe

wheels.Donotsetthespringparkingbrakes

untilyouhaveverifiedthebrakedrumiscool

tothetouch.

Mounting Bolts

Clevis and pin

Slackadjuster

Push rod

Diaphragmreturnspring

Diaphragm

Spring parkingbrake chamber

Dustcap

Parking brake springService brake chamber

Spring Parking Brakes AppliedBrakes On


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Air Brake Manual

Mechanical Release (Caging)

Somespringparkingbrakescanbereleased

mechanicallybywindingthemofforcaging

them.Cagingmeansthebrakesarebeingreleased.

Thisisachievedwithaboltthatrunsthrough

thecentreofthechamberbody,whichisturned

tocompressthespring.Itmaybenecessaryto

firstremovealockplateandstudtogainaccess

totheheadofthebolt.Othertypeshaveadust

capthatmustfirstberemovedandaboltinserted.

Insomecases,aspecialwrenchisrequired.

Instructiononhowtocageisusuallyonthe

bodyoftheparkingbrakechamber.Ifallairislost

andthevehiclehastobetowed,theparkingbrakes

canbereleasedbycagingthem.Alwaysblockthe

wheelswhencagingtheparkingbrakespring.

Warning

Springparkingbrakechambersshouldneverbe

disassembledwithoutfirstcompressingthespring

withacagingbolt.Thesespringsareunderextreme

pressureandcouldcauseseriouspersonalinjury

ifdisassemblyisattemptedbyanyonenotexperienced

inservicingtheseunits.Disassemblyofaspring

brakechambershouldonlybepreformedby

aqualifiedmechanicortechnician.


1. Whatismeantbycompoundingthebrakes?

2. Whyarespringbrakesareliabletype

ofparkingbrake?

3. Howareparkingbrakesheldinthe

releasedposition?

4. Whatarethefunctionsofthecabmounted

parkingbrakecontrolvalve?

5. Willparkingbrakesapplyautomaticallyinallbrakingsystems?

6. Whatisthereasonforreleasingtheparkingbrake

beforemakingafullbrakeapplicationtest?

7. Whymustyoubecarefulparkingavehiclewith

overheatedbrakes?

8. Howcansometypesofparkingbrakesbereleased

withouttheuseofairpressure?

9. Whatisthedangerofdisassemblingaspring

parkingbrakeunit?

Mounting Bolts

Springparkingbrake

chamber


Clevis and pin

Slackadjuster

Push rod

DiaphragmDiaphragm returnspring

Caginbolt

Parking brakespring

Parking Brake Spring CagedBrakes O


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TrailerSystem

Section5


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38 Air Brake Manual

Uptothispoint,thesystemdiscussedistheairbrake

systemofatruckortractor.Ifatrailerwascoupled

toatruckortractor,thebrakesofthetrailerwould

havetobeoperatedfromthetruckortractor.

Inthefollowingpagesthepowerunitofacombination

vehiclewillbereferredtoasatractor.

Glad Hands

Thistermreferstothecouplingdeviceusedtoconnect

thecontrol(service)andsupply(emergency)linesofthe

trailertothetractor.Thesecouplersconnecttogether

andlockinposition.Theyhaverubbergasketsthat

preventairfromescapingattheconnections.

Beforeconnectionismade,couplersshouldbeclean

andfreeofdirtandgrit.Whenconnectingtheglad

hands,startwiththetwogasketstogetherandthe

couplersata90angletoeachother.Aquicksnap

downwardswilljoinandlockthecouplers.Deadendcouplersshouldbeusedwheneverthevehicleis

drivenwithoutatrailertopreventwateranddirtfrom

enteringthecouplerandlines.

Iftheunitisnotequippedwithdeadendcouplers,

thegladhandofthecontrol(service)linecanbelocked

tothegladhandofthesupply(emergency)lineto

keepwateranddirtfromenteringthelines.

Thecleanertheairsupplyiskept,thelesschance

ofbrakeproblems.

Gladhandsandlinesshouldalsobesecuredtoprevent

thelinesfromchafingagainstvehiclecomponentsor

bouncingoffthevehicle.Thiscouldseriouslydamage

thegladhandsorlines.

Application Line

Theapplicationlineisreferredtoasacontrol(service)

line.Thislineisconnectedtothefootandhandvalve.

Whenthedriverdepressesthefootvalvetreadle

applicationairwillbedeliveredtothetractorbrake

chambersandtothetrailerbrakechambers.Whenthe

driverreleasesthefootvalvetreadle,theapplicationair

tothetrailerbrakechambersmustreturntothefoot

valvetobeexhaustedtotheatmosphere.

Thedisadvantagesofthissystemare:

ifthetrailerbrokeawayfromthetractor,thetrailerwouldnothavebrakes.

ifthecontrol(service)linepartedorruptured,

thetrailerbrakeswouldnotbeapplied,andthe

applicationairwouldbelostfromthetractor

ifthebrakeswereapplied.

iftheairpressureinthereservoirsislost,

therewouldbenowaytoapplythebrakes

ofthetractororthetrailer.

thetrailerbrakescannotbeappliedindependently

fromthetractorandthereisnowaytosetthe

trailerbrakeswhencouplingtothetractor.

theapplicationandreleaseofthetrailerbrakes

wouldbeslowerthanthoseofthetractor.

Thesedisadvantagesareovercomebytheaddition

ofthesupply(emergency)lineandvalvesdiscussed

inthefollowingpages.

Theillustrationshowsthepipingofaunitwithbrakes

applied,similartothetandemaxlesofthetractor.

Alsowithbrakesapplied,thetrailerhastandemaxles

equippedwithbrakechambers.

TheapplicationlinehasaTinsertedbetweenthefoot

valve(31)andthetractorsrelayvalve(13).Anairline

hasbeenconnectedfromthisTtothetrailerbyasetofcouplers(gladhands)(20).

Air line

Rubber seal

Air line


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Air Brake Manual

Thepurposeofthetrailerbrakehandvalve(30)isto

allowthedrivertocontrolindependentlytheamount

ofapplicationairpressuretobedirectedtothetrailer

brakes.Italsoprovidesamethodofapplyingthetrailer

brakeswhencouplingthetrailertothetractor.

Thevalve,alsoallowsthedrivertoapplythetrailer

brakesindependentlyofthetractor.Theamountof

applicationairpressuredelivereddependsonthe

amountthevalveisopenedbythedriver.(Itcannot

exceedthereservoirairpressure.)Somevalvesare

equippedwithselfreturninghandles.

13 20T

31

Application Line

30

31

Trailer Brake Hand Valve


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40 Air Brake Manual

Note: Thetrailerbrakehandvalveisnottobeusedfor

parking,asairmaybleedoffiftheengineisstopped

orthehandvalvemovestothereleasedposition.

Thedriverhasappliedthebrakesbyusingthefoot

valve(31).Applicationairisdirectedtothebrake

chambersofthetractorandtothetrailerbrakes

throughatwowaycheckvalve(26).Theshuttlehas

shiftedtothelowpressureside,closingoffanyair

TwoWay Check Valves

Thepurposeofatwowaycheckvalve(26)istodirect

airflowintoacommonlinefromeitheroftwosources.

Thisvalvewillpermitairtoflowfromthesourcethat

issupplyingthehigherapplicationpressures.The

shuttlewillshiftsothatthehigherpressurewillbe

directedtothetrailerthroughthecontrol(service)line.Thisvalveislocatedbetweenthefootoperatedvalve

andthehandoperatedvalve.

flowtowardthehandvalveside.Thehandvalve

(30)isintheclosedpositionandequalpressure

isbeingappliedtothebrakechambersofthe

tractorandthebrakechambersofthetrailer.

30 26

31

Air romhand valve

Air romoot valve

To trailer

Shuttle

Foot Valve Application


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Air Brake Manual

Inthisillustrationwiththefootvalve(31)released

andthehandvalve(30)opened,applicationairis

directedfromthehandvalvethroughatwowaycheck

valve(26),tothebrakechambers.Thetwowaycheck

valveinthisapplicationhasshiftedtothelowpressure

side,closingoffanyairflowtowardthefootvalveside.

Anytimeatrailerbrakeapplicationismadeusing

thehandvalve,thedrivermaydepressthefoot

valvetreadle.Ifthefootvalveapplicationisofa

higherpressurethanthatofthehandvalve,the

twowaycheckvalvewillshifttothelowerpressure

side,allowingthehigherpressuretobedirected

tothetractorandtrailerbrakes.Duringafoot

valveapplication,ifthedrivermakesahandvalve

applicationofahigherairpressure,thetwoway

checkvalvewilldirectthehigherhandvalveair

pressuretothetrailerbrakes.

Althoughthetrailerbrakesmaybeapplied

independentlybymeansofthehandvalve,the

maximumapplicationpressurecanonlybethe

sameas,orslightlylessthan,reservoirpressure.

Tractor Protection System

Atractorprotectionsystempreventstotallossofair

fromthetractorifthetrailerbreaksaway,orifthe

connectingairlinesbetweentractorandtrailer

becomeseparatedorruptured.Thetractorprotection

systemconsistsoftwovalves:thetractorprotection

valveandthetrailersupplyvalve.Othernamesfor

thetrailersupplyvalvearetrailersupplyvalve

andemergencyvalve.Therearetwotypesoftrailersupplyvalves.

Themostcommonistheautomatictrailersupply

valve.Thisisaspringloadedvalvethatisopened

manuallyandheldopenbyairsystempressure.

Onoldervehiclesyoumayfindamanualtrailer

supplyvalve,whichmaybeatoggletypeswitch

orapush/pulltypevalve.

Tounderstandthefunctionofthetrailersupply

valveandthetractorprotectionvalveinthesystem,

itisimportanttounderstandhowtheyoperate.

30 26

31

Hand Valve Application


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42 Air Brake Manual

Tractor Protection Valve

Atractorprotectionvalve(24)isnormallylocated

neartherearofthetractorcabandhastwoairlines

runningtoit,onefromthetrailersupplyvalve(28)

andtheotherfromthetwowaycheckvalve(26)

fedbythehandorfootvalve.

Italsohastwoairlines:onegoingtothesupply

(emergency)line(21),andonegoingtothecontrol

(service)line(22).Thetractorprotectionvalve

isspringloadedandrequiresapproximately45psiin

thesupply(emergency)linetoopenthevalve.This

allowscontrol(service)airtopassthroughthecontrol

(service)linetothetrailerwhilemakingabrake

application.Whenairlinesfromthetractorarecoupled

toatrailer,theopeningorclosingofthetrailersupply

valveopensorclosesthetractorprotectionvalve.

Disconnectingthesupply(emergency)linebetween

thetractorandtrailerwhilethetrailerischargedwill

causeanimmediatelossofpressureinthesupply

(emergency)line.Thesuddenlossofairpressurein

thelinewillcausethetrailersupplyvalvetoclose,

andwithnoairpressureinthelinethetractor

protectionvalvewillclose.Whenthetractorprotection

valueclosesitwillstoptheairflowtothecontrol

(service)line.

Intheeventofacontrol(service)lineruptureordisconnectionbetweenthetractorandtrailernoaction

orairlosswilltakeplaceuntilabrakeapplicationis

made.Serviceairwillbelostoutofthedisconnectedor

rupturedlinecausingthetractorsairpressuretodrop.

Atapproximately45psithetrailersupplyvalvewill

closecausinganemergencyapplicationofthetrailer

brakesandthetractorprotectionvalvetoclose.This

willstopthelossofserviceairatthedisconnectedline.

Thetractorprotectionvalvealsoprotectsthe

tractorsserviceairfrombeinglostduringnormal

brakeapplicationswhileoperatingthetractor

withoutatrailer.

To the supply(emergency)glad hand

To the control(service) glad

hand

Tractor Protection System (Open)(Trailer Charged)

Air romoot valve

Air romhand valve

Twowaycheck valve

(26)

Control(service)line (22)

Supply(emergency)

line (21)

Tractor protection valve (24)

Trailer supply valve (mounted in cab) (28)

Air romreservoir


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Totesttheproperoperationofthetractorprotection

valve,ensurethetractorspringparkingbrakesare

appliedandthewheelsareblocked.Verifythesystem

isatfullpressureandthetrailersupplyvalveisopen

(trailercharged).Exitthecabanddisconnectthe

control(service)andensurethatthereisnoairloss

fromthecontrol(service)linegladhand.Enterthecab,

ensurefullreservoirpressurebetween115psiand135

psiandopenthetractorspringparkingbrakecontrol

valve.Shutofftheengineandmakeafullservicebrake

application.Airshouldescapefromthecontrol(service)

linetonolowerthan20psi,thetractorprotectionvalve

willcloseandtheairlosswillstop.Releasetheservice

brakeapplication.Closethetractorspringparking

brakecontrolvalveandensurethetrailersupplyvalve

isclosed,ifnot,closemanually.Starttheengineand

rebuildairpressure.Withairpressureatorabove90

psi,makeatrailerservicebrakeapplicationwithtrailer

handvalve.Noairshouldescapefromthedisconnected

control(service)line.Releasethebrakeapplication.

Exitthecabandreconnectthecontrol(service)line

gladhand.

Thisteststheproperopeningandclosingoperation

ofthespringportionofthetractorprotectionvalve.Atractorprotectionvalvewithabrokenreturnspring

willnotclosethevalveandwillcauselossofairduring

normalbrakeapplicationsifoperatingthetractor

withoutatrailer.

Trailer Supply Valve

Thetrailersupplyvalve(28)(usuallyaredoctagonal

button)ismountedinthecabofthetractor.Thedriver

opensthevalvebypushingorpullingthebutton,

dependingonthetypeused.

Openingthevalvepermitsreservoirairpressure

toflowthrough.Thisairpressureispipedtothe

tractorprotectionvalveandthentothesupply

(emergency)line.

Thevalveisspringloadedandwillbeheldintheopenpositionwhensufficientpressureis

reached.Iftheairpressuredropstoarange

between45and20psi,thevalvewillclose

automaticallybyspringpressure.Thedriver

canclosethevalvemanually,applyingthe

trailerspringparkingbrakes.

Tractor Protection System (Closed) (Trailer Not Charged)

Air rom

oot valve

Air romhand valve

Twowaycheck valve

(26)

Control(service)line (22)

Supply(emergency

line (21)

Tractor protection valve (24)

Trailer supply valve (mounted in cab) (28)

Air romreservoir


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44 Air Brake Manual

Automatic Trailer Supply Valve System

Thediagrambelowillustratesairbeingpipedfrom

thedryreservoirlinetothetrailersupplyvalve(28).

Thetractorprotectionvalve(24)isfedbytwolines:

onefromthetrailersupplyvalve(28)andonefrom

Inthediagramstheupperlineisthecontrol

(service)line(22)andthelowerlineisthesupply(emergency)line(21).

Illustratedisatractorequippedwithatrailersupply

valve(28)andatractorprotectionvalve(24).

thetwowaycheckvalve(26).Leadingofffromthe

tractorprotectionvalvearetwolines,eachwith

agladhandcoupler(20).Thesetwolinesarereferred

toasthecontrol(service)line(22)andthesupply

(emergency)line(21).

Thetrailerisnotcoupledandthetractorisbeing

operatedalone(bobtailing).Thedriverhasnotopenedthetrailersupplyvalve(28)andthehand

valve(30)isclosed.

28 26

24

22 21

30 28

31

26

24

20

22 21

20


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Air Brake Manual

Inthisillustration,thedriverhasmadeabrake

applicationwiththefootvalve(31)andapplication

airisbeingdeliveredtothetractorbrakechambers.

Thetwowaycheckvalve(26)hasshiftedtothelow

Ifthedriver,bymistake,appliedthehandvalve(30)

withthetrailerdisconnected,theapplicationairdirectedtothetractorprotectionvalvewouldalsobe

pressureside,allowingapplicationairtoreach

theclosedtractorprotectionvalve(24).

Thereisnoairlossfromthetractorthrough

thedisconnectedgladhands(20).

deadended.Again,noairlosswouldoccurifthetraile

supplyvalve(28)isintheclosedposition.

31

26

24

30 28

20

31

26

2430 28

20


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46 Air Brake Manual

Inthisillustration,thetrailerhasbeencoupled

tothetractor,andthecontrol(service)andsupply

(emergency)lineshavebeencoupledusingglad

hands(20).

Thetrailerunithasareservoir(16)installed.This

reservoirwillprovideavolumeofairnearthetrailer

brakechambersfornormaloremergencybraking.

Thetrailerreservoirisfittedwithadrainvalve(6),

thesameasatractorreservoir.

Arelayemergencyvalve(39)ismountedonthetrailerreservoirortothetrailerframenearthe

brakechambers.Therelayemergencyvalveserves

threemainfunctionsinthesystem:

1.Itrelaysairfromthetrailerreservoirtothe

trailerbrakechambersduringabrakeapplication.

Thispartofthevalveoperatesliketherelayvalve

previouslydiscussed.Italsoprovidesaquick

releaseofthetrailerbrakes.

2.Itdirectstrailerreservoirairpressuretothetrailer

brakes,causinganemergencyapplicationofthe

brakes.Thisactionwilloccurautomaticallyinthe

eventofrupturedorseparatedsupply(emergency)

airlinebetweenthetractorandtrailer,orlossofair

pressurefromthetractor.Arupturedcontrol(service)

linewouldnotcauseanemergencybrakeapplication.

Ifabrakeapplicationwasmadetherewouldbea

rapidlossoftractorreservoirair.Ifthebrake

applicationwasheldtheairpressurewoulddrop

lowenoughtocauseanemergencyapplication.Thedriveratanytimemayoperatethecabmounted

trailersupplyvalve(28)tocauseanemergency

applicationofthetrailerbrakes.

3.Ithasaonewaycheckvalvewhichstopsair

pressureinthetrailerreservoirfromgoingback

tothetractor.

20

16

6

39

28

Tractor and Trailer Coupled


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Air Brake Manual

Intheillustration,thecompressorhasraisedthe

reservoirairpressuretomaximum.Thedriverhasopenedthetrailersupplyvalve(28)to

allowthereservoirairpressuretobedirectedthrough

thetractorprotectionvalve(24)tothetrailer.Theair

pressurepassesthroughtherelayemergencyvalve(39)

Thisandthenextillustrationaretoonlyshowwhich

brakecomponentsandlinesareusedforafootvalve

andhandvalveapplication.

Applicationwiththefootvalve(31)inorange.

Applicationwiththehandvalve(30)inbrown.

Orangeandbrownareusedtoshowwhereairis

appliedanddoesnotrepresentthesourceoftheair.

Whenthefootvalveisdepressed,theapplicationair

tothereservoironthetrailer.Pressurewillbuild

upinthetrailerreservoirtothesamepressureasthereservoirsonthetractor.Thisisknownascharging

thetrailersystem.Thetrailersupplyvalveshould

onlybeopenwhenthetractorpressurehasbuiltup

toapproximately90psi,dependingonthevalve.

willbeappliedtothetractorbrakesandthetrailer

brakestogether.Aspreviouslyexplained,the

twowaycheckvalve(26)hasshifted,andapplication

airisbeingdirectedthroughthetractorprotection

valve(24)tothecontrol(service)line.Ifthefootvalve

isreleasedandthehandvalveisapplied,theshuttle

inthetwowaycheckvalvewouldshiftandapplication

airwouldapplythetrailerbrakesonly.

31

24

30

26

39

2428

Charging the Trailer System

Foot or Hand Valve Brake Application


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48 Air Brake Manual

Applicationairdirectedfromeitherthefootorhand

valvecausescontrolpressuretomovethroughthe

control(service)lineandactontherelayemergencyvalve(39).Thiscontrolpressurewillcausetherelay

emergencyvalvetodirectairfromthetrailerreservoir

(16)tothetrailerbrakechambers(14).Thepressure

directedbytherelayemergencyvalvetothetrailer

brakechamberswillbethesameasthepressure

directedtothetractorbrakechambers.Insuch

asystem,brakelagtimehasbeenminimized

withtheadditionofthetrailerreservoirandrelay

emergencyvalve.

Releaseofthefootorhandvalvestopstheflowof

applicationair.Therelayportionofthevalvereturns

toitsoriginalposition,stoppingtheflowofairpressure.Theexhaustingportsofthevalveexhaust

theairpressurefromthebrakechambers,releasing

thebrakes.Inthissystem,thebrakesofboththe

tractorandtrailercanbereleasedquickly.

Caution:Trailerbrakesmustnotbeusedtohold

aparkedvehiclethatisleftunattended.Lossof

pressurewillresultinlossofbrakes.Alwaysset

theparkingbrake.

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Emergency Application


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Ifatrailerthatisnotequippedwithspringparking

brakesweretoseparatefromthetractor,thecontrol

(service)line(22)andthesupply(emergency)line(21)

woulddetachfromthetractor.Thesuddenlossofair

pressureinthesupply(emergency)linewilltrigger

therelayemergencyvalve(39)whichcausesthetrailer

reservoir(16)todumpitsairdirectlytothetrailer

brakechambers(14).Thisplacesthetrailerbrakesinto

emergencyapplication.Thecheckvalveintherelay

Supply (Emergency) Line Rupture

Aruptureofthesupply(emergency)line(21)oran

uncouplingofthesupply(emergency)linegladhands

willtriggerthesameactionasdescribedabove.

Control (Service) Line Rupture

Ifthecontrol(service)line(22)isrupturedor

disconnected,noactionwilltakeplaceuntilabrake

applicationismade.Whenitismadebyeitherthe

footorhandvalve,theairlossfromthecontrol

(service)linewilllowertheairpressureinthetractors

reservoirsquickly,dependingontheamountofbraking

demanded.Thisairlosswilleventuallyactivatethe

tractorprotectionsystemwhichinturnwillexhaust

thetrailersupply(emergency)lineandcausethe

trailerrelayemergencyvalvetoapplythetrailerbrakes.

Itshouldbenotedthatanyproblemthatcauses

emergencyvalvewillclose,preventinganybleedback

pressurefromthetrailerreservoir.Thelossofpressure

inthetrailersupply(emergency)linewillcausethe

tractorprotectionsystemtosealoffthetractorleaving

sufficientairpressuretobrakethetractor.

Thetrailerbrakeswillremainapplieduntileitherthe

pressureinthetrailerreservoirandlinesisdrainedoff,orthesupply(emergency)lineisrepairedand

thesystemisrecharged.

aseveredropinreservoirpressureonthetractor

system,willcausethelowwarningdeviceto

signalthedriver.

Inthefollowingillustration,thecontrol(service)line(22)hasrupturedandthedriverismakingabrake

applicationwiththefootvalve(31).Thetractorwill

havebrakesappliedbutthetrailerwillhavenobrakes

Ifthebrakeapplicationisheld,theairpressureinthe

tractorsystemwilllowertoadangerouslylowlevelan

thenthetractorprotectionsystemwillplacethetraile

brakesintoanemergencyapplication.

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Ruptureofthecompressordischargelinewouldresult

inlossofairpressurefromthesupply/wetreservoir(5).Whentheairpressureinthesupply/wetreservoir(5)

ofthetractorfallsbelowthewarninglevel,dueto

acompressorfailureorexcessiveleakageonthe

tractor,thewarningdeviceswillstarttooperate.

Inthediagram,theonewaycheckvalve(7)has

preventedthereservoirairpressureintheprimary/dry

reservoir(8)fromescapingbacktothesupply/wet

reservoirandtherupturedline.

Thereissufficientreservoirairpressureinthe

primary/dryreservoirforalimitednumberofbrakeapplicationstostopthevehiclebeforethespring

parkingbrakesareactivated.(Thiswilldependonhow

thespringparkingbrakesarepipedinthesystem.)

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Control (Service) Line Rupture

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Loss o Reservoir Air Pressure


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Inthisillustrationthepressurehasbeenloweredto

approximately4520psiandthetractorprotection

systemhasclosedautomatically,placingthetrailer

brakesintoanemergencyposition.Also,thespring

parkingbrakessystemhashadtheairpressure

releasedactivatingthespringparkingbrakes.

Manual Trailer Sup

air brakes manual

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