Brakes Ch08

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  • After studying Chapter 8, the reader should be able to:

    1. Prepare to take the Brakes (A5) ASE certification test content areaA (Hydraulic System Diagnosis and Repair).

    2. Explain how to bench bleed a master cylinder.

    Bleeder valve (p. 131)Brake bleeding (p. 131)Gravity bleeding (p. 135)Power bleeding (p. 137)Pressure bleeding (p. 137)

    Chapter 8

    BRAKE BLEEDINGMETHODS AND

    PROCEDURES

    BRAKE BLEEDINGMETHODS AND

    PROCEDURES

    3. Describe the proper brake bleeding sequence.

    4. Describe the single stroke manual brake bleeding procedure.

    5. Discuss how to gravity bleed the hydraulic brake system.

    6. List the steps needed to perform a pressure bleed procedure.

    Reverse fluid injection (p. 139)Single stroke bleeding method (p. 134)Surge bleeding (p. 139)Vacuum bleeding (p. 134)

    OBJECTIVES

    KEY TERMS

  • Brake Bleeding Methods and Procedures 131

    FIGURE 8-1 Always clamp a master cylinder in a vise by the mount-ing flange to prevent distortion of the cylinder bore.Bench bleeding tubes routethe fluid from the outlet back into the reservoir.

    HOLE

    FIGURE 8-2 Typical bleeder valve from a disc brake caliper. The ar-rows point to the taper section that does the actual sealing. It is this taper thatrequires a shock to loosen. If the bleeder is simply turned with a wrench, thebleeder usually breaks off because the tapered part at the bottom remains ad-hered to the caliper or wheel cylinder. Once loosened, brake fluid flows aroundthe taper and out through the hole in the side of the bleeder valve.

    BRAKE BLEEDING

    Brake bleeding is removing any trapped air from the hy-draulic system. Air can get into the hydraulic system when-ever any hydraulic brake line or unit is opened. Air can also bedrawn into the hydraulic system through small holes or loosebrake line connections during the release of the brake pedal.A common source of air in the brake system of this type canoccur through very small holes in rubber flexible brake lines.Another source of air in the braking system is through the ab-sorption of moisture by the brake fluid. When moisture is ab-sorbed, the boiling point of the brake fluid is reduced. Duringsevere braking, the heat generated can cause the brake fluidto boil and create air bubbles in the hydraulic brake system.Air eventually travels to the highest part of the brake system,if not restricted by pressure control valves.

    BLEEDING THE MASTERCYLINDER

    Whenever the master cylinder is replaced or the hydraulic sys-tem has been left opened for several hours, the air may have tobe bled from the master cylinder. Bleed the master cylinder onthe bench before installing it on the vehicle. See Figure 8-1.

    If bleeding the master cylinder after working on the hy-draulic system, follow these steps:

    Step 1 Fill the master cylinder with clean brake fluidfrom a sealed container up to the recommendedfull level.

    Step 2 Have an assistant slowly depress the brake pedal asyou crack open the master cylinder bleed screw

    starting with the section closest to the bulkhead. Itis very important that the primary section of themaster cylinder be bled before attempting to bleedthe air out of the secondary section of the mastercylinder. Before the brake pedal reaches the floor,close the bleeder valve.

    HINT: A proper manual bleeding of the hydraulicsystem requires that accurate communications oc-cur between the person depressing the brake pedaland the person opening and closing the bleedervalve(s). The bleeder valve (also called a bleedvalve) should be open only when the brake pedal isbeing depressed. The valve must be closed whenthe brake pedal is released to prevent air from be-ing drawn into the system.

    Step 3 Repeat the procedure several times until a solid flowof brake fluid is observed leaving the bleeder valve.If the master cylinder is not equipped with bleedervalves, the outlet tube nuts can be loosened instead.

    BRAKE BLEEDER VALVELOOSENING METHODS

    Attempting to loosen a bleeder valve often results in breaking(shearing off) the bleeder valve. Several of these service proce-dures can be tried that help prevent the possibility of breakinga bleeder valve. Bleeder valves are tapered and becomewedged in the caliper on the wheel cylinder housing. SeeFigures 8-2 and 8-3.

    All of these methods use shock to break the taper andto loosen the stuck valve.

  • 132 CHAPTER #

    TechTipDO IT RIGHTREPLACE THEBRAKE FLUID

    Often, used brake fluid looks like black coffee or coffeewith cream. Both conditions indicate contaminated ormoisture-laden brake fluid that should be replaced. Thefollowing steps will help assure a complete brake fluidchange:

    Step 1 Remove the old brake fluid from the mas-ter cylinder using a suction bulb. (Disposeof this old brake fluid properly.)

    Step 2 Fill the master cylinder with new cleanbrake fluid from a sealed container.

    Step 3 Bleed each wheel brake until the brakefluid is clean.

    CAUTION: Do not allow the master cylinder to run out of brakefluid. Recheck and refill as necessary during the bleeding process.

    This brake fluid replacement will fully restore thebrake hydraulic system to as-new condition and help pro-tect the system from rust and corrosion. Replacing onlythe friction pads and/or linings is not a complete andthorough brake system service. Customers should be ed-ucated as to the importance of this service procedure.

    Air Impact Method

    Use a 6-point socket for the bleeder valve and use the neces-sary adapters to fit an air impact wrench to the socket. Applysome penetrating oil to the bleeder valve and allow it to flowaround the threads. Turn the pressure down on the impactwrench to limit the force. The hammering effect of the impactwrench loosens the bleeder valve without breaking it off.

    Hit and Tap Method

    Step 1 Tap on the end of the bleeder valve with a steelhammer. This shock often breaks the taper at thebase of the bleeder valve. The shock also breaksloose any rust or corrosion on the threads.

    Step 2 Using a 6-point wrench or socket, tap the bleedervalve in the clockwise direction (tighten).

    Step 3 Using the same 6-point socket or wrench, tap thebleeder valve counter-clockwise to loosen and re-move the bleeder valve.

    NOTE: It is the shock of the tap on the wrench thatbreaks loose the bleeder valve. Simply pulling on thewrench often results in breaking off the bleeder.

    Step 4 If the valve is still stuck (frozen), repeat Step 1through Step 3.

    Air Punch Method

    Use an air punch near the bleeder valve while attempting toloosen the bleeder valve at the same time. See Figure 8-4.

    VALVE

    INLETOUTLET

    WHEEL CYLINDER

    BLEEDSCREW

    OUTLETPORTS

    MASTER CYLINDER

    BLEEDSCREW

    OUTLET

    OUTLET

    CALIPER

    INLET

    FIGURE 8-3 Typical bleeder locations. Notethat the combination valve and master cylinder showndo not have bleeder valves; therefore,bleeding is accom-plished by loosening the brake line at the outletparts. (Courtesy of Allied Signal Automotive Aftermarket)

  • Brake Bleeding Methods and Procedures 133

    FIGURE 8-4 Using an air punch next to the bleeder valve to helpbreak the taper on the bleeder valve.

    BLEEDER VALVE BRAKE LINE

    FIGURE 8-5 Most vehicle manufacturers recommend starting thebrake bleeding process at the rear wheel farthest from the master cylinder.

    The air punch creates a shock motion that often loosensthe taper and threads of the bleeder valve from the caliper orwheel cylinder. It is also helpful to first attempt to turn thebleeder valve in the clockwise (tightening) direction, thenturn the bleeder in the counterclockwise direction to loosenand remove the bleeder valve.

    Heat and Tap Method

    Heat the area around the bleeder valve with a torch. The heatexpands the size of the hole and usually allows the bleeder tobe loosened and removed.

    CAUTION: The heat from a torch will damage the rubber seals inside thecaliper or wheel cylinder.Using heat to free a stuck bleeder valve will requirethat all internal rubber parts be replaced.

    Wax Method

    Step 1 Heat the bleeder valve itself with a torch. The heatcauses the valve itself to expand.

    Step 2 Remove the heat from the bleeder valve. As thevalve is cooling, touch paraffin wax or candle waxto the hot valve. The wax will melt and run downaround the threads of the bleeder valve.

    Step 3 Allow the bleeder valve to cool until it can be safelytouched with your hand. This assures that the tem-perature is low enough for the wax to return to asolid and provide the lubricating properties neces-sary for the easy removal of the bleeder valve.Again, turn the bleeder valve clockwise beforeturning the valve counterclockwise to remove.

    BLEEDING SEQUENCE

    After bleeding the master cylinder, the combination valveshould be bled if equipped. Follow the same procedure aswhen bleeding the master cylinder, being careful not to allowthe master cylinder to run dry.

    NOTE: The master cylinder is located in the highest section of the hydraulicbraking system.Some master cylinders are equipped with bleeder valves.Allmaster cylinders can be bled using the same procedure as that used forbleeding calipers and wheel cylinders.If the master cylinder is not equippedwith bleeder valves, it can be bled by loosening the brake line fittings at themaster cylinder.

    Check the level in the master cylinder frequently andkeep it filled with clean brake fluid throughout the brakebleeding procedure.

    For most rear-wheel-drive vehicles equipped with afront/rear split system, start the bleeding with the wheel far-thest from the master cylinder and work toward the closest.See Figure 8-5.

    For most vehicles, this sequence is as follows.

    1. Right rear2. Left