Brakes Ch08

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

1. Prepare to take the Brakes (A5) ASE certification test content area

“A” (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 BLEEDING

METHODS AND

PROCEDURES

BRAKE BLEEDING

METHODS 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 MASTER

CYLINDER

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 recommended“full” 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 VALVE

LOOSENING 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 RIGHT—

REPLACE THE

BRAKE 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

INLET

OUTLET

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)


FIGURE 8-4 Using an air punch next to the bleeder valve to help“break 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 the

caliper 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 hydraulic

braking system.Some master cylinders are equipped with bleeder valves.All

master cylinders can be bled using the same procedure as that used for

bleeding calipers and wheel cylinders.If the master cylinder is not equipped

with bleeder valves, it can be bled by loosening the brake line fittings at the

master 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 rear3. Right front4. Left front

NOTE: If the vehicle has two wheel cylinders on one brake, bleed the up-

per cylinder first.

For vehicles equipped with a diagonal split section orequipped with ABS, follow the brake bleeding procedure rec-ommended in the service information for the vehicle.

134 CHAPTER 8

FIGURE 8-6 Bleeding brakes using clear plastic tubing makes it easyto see air bubbles.Submerging the hose in a container of clean brake fluid helpsensure that all of the air will be purged by the system.

MANUAL BLEEDING

Manual bleeding uses hydraulic pressure created by the mas-ter cylinder to pump fresh fluid through the brake system. Thismethod is called the single stroke bleeding method.

It is extremely important when manually bleeding a brakesystem that the pedal be applied and released slowly and gently.Rapid pedal pumping can churn up the fluid and reduce the sizeof trapped air bubbles, making them more difficult to bleed fromthe system. There are, however, special situations where rapidpedal pumping may be helpful in brake system bleeding. Theseare covered in the “Surge Bleeding” section later in the chapter.

Manual bleeding requires an assistant to apply and releasethe brake pedal, a bleeder screw wrench, approximately twofeet of clear, plastic hose with an inside diameter small enoughto fit snugly over the bleeder screws, and a clear jar partiallyfilled with clean brake fluid. To manually bleed the brake sys-tem, follow these steps.

1. Discharge the vacuum or hydraulic power booster (ifequipped) by pumping the brake pedal with the ignitionOFF until the pedal feels hard.

2. Fill the master cylinder reservoir with new brake fluidand make sure it remains at least half full throughout thebleeding procedure.

3. Attach the plastic hose over the bleeder screw of the firstwheel cylinder or caliper in the bleeding sequence, andsubmerge the end of the tube in the jar of brake fluid. SeeFigure 8-6.

4. Loosen the bleeder screw approximately one-half turn,and have an assistant slowly depress the brake pedal. Airbubbles leaving the bleeder screw will be visible in thehose to the jar.

5. Tighten the bleeder screw, then have your assistantslowly release the brake pedal.

6. Wait at least 15 seconds to allow time for any small bub-bles to form into larger bubbles. See the Tech Tip called,“Tiny Bubbles.”

7. Repeat steps 4 and 5 until no more air bubbles emergefrom the bleeder.

8. Transfer the plastic hose to the bleeder screw of the nextwheel cylinder or caliper in the bleeding sequence, andrepeat steps 4 through 7. Continue around the vehicle inthe specified order until the brakes at all four wheelshave been bled.

NOTE: Make certain all the brake components such as calipers and wheel

cylinders are correctly installed with the bleeder valve located on the high-

est section of the part.Some wheel cylinders and calipers (such as many Ford

calipers) can be installed upside down! This usually occurs whenever both

front calipers are off the vehicle and they accidentally get reversed left to

right. If this occurs, the air will never be completely bled from the caliper.

VACUUM BLEEDING

Vacuum bleeding uses a special suction pump that attachesto the bleeder screw. The pump creates a low-pressure area atthe bleeder screw, which allows atmospheric pressure to forcebrake fluid through the system when the bleeder screw isopened. Vacuum bleeding requires only one technician. Tovacuum bleed a brake system follow these steps.

1. Fill the master cylinder reservoir with new brake fluidand make sure it remains at least half full throughout thebleeding procedure.

2. Attach the plastic tube from the vacuum bleeder to thebleeder screw of the first wheel cylinder or caliper in thebleeding sequence. See Figure 8-7. If necessary, use one ofthe adapters provided with the vacuum in the catch bottle.

3. Operate the pump handle to create a partial vaccum inthe catch bottle.

4. Loosen the bleeder screw approximately one-half turn.Brake fluid and air bubbles will flow into the bottle.When the fluid flow stops, tighten the bleeder screw.

5. Repeat steps 3 and 4 until no more air bubbles emergefrom the bleeder.


TechTipTINY BUBBLES

Do not use excessive brake pedal force while bleedingand never bleed the brake with the engine running! Theextra assist from the power brake unit greatly increasesthe force exerted on the brake fluid in the master cylin-der. The trapped air bubbles may be dispersed into tinybubbles that often cling to the inside surface of the brakelines. These tiny air bubbles may not be able to be bledfrom the hydraulic system until enough time has allowedthe bubbles to reform. To help prevent excessive force,do not start the engine. Without power assistance, thebrake pedal force can be kept from becoming excessive.If the dispersal of the air into tiny bubbles is suspected,try tapping the calipers or wheel cylinders with a plastichammer. After this tapping, simply waiting for a periodof time will cause the bubbles to re-form into larger andeasier-to-bleed air pockets. Most brake experts recom-mend waiting 15 seconds or longer between attempts tobleed each wheel. This waiting period is critical and al-lows time for the air bubbles to form.

HINT: To help prevent depressing the brake pedal down too far,some experts recommend placing a 2 × 4 in. board under the brakepedal.This helps prevent the seals inside the master cylinder from trav-eling over unused sections inside the bore that may be corroded or rusty.

VACUUMPUMP

ESCAPINGAIR BUBBLES

FLUIDCATCHBOTTLE

OPEN BLEEDERVALVE

FIGURE 8-7 Vacuum bleeding uses atmospheric pressure to forcebrake fluid through the hydraulic system.

6. Transfer the vacuum bleeder to the bleeder screw of thenext wheel cylinder or caliper in the bleeding sequence,and repeat steps 3 and 4. Continue around the vehicle inthe specified order until the brakes at all four wheelshave been bled.

GRAVITY BLEEDING

Gravity bleeding is a slow, but effective, method that willwork on many vehicles to rid the hydraulic system of air. Theprocedure involves simply opening the bleeder valve and wait-ing until brake fluid flows from the open valve. Any air trappedin the part being bled will rise and escape from the port whenthe valve is opened. It may take several minutes before brakefluid escapes. If no brake fluid comes out, remove the bleedervalve entirely—it may be clogged. Remember, nothing but airand brake fluid will be slowly coming out of the wheel cylin-der or caliper when the bleeder valve is removed. Do not

press on the brake pedal with the bleeder valve out whilegravity bleeding.

Gravity bleeding works because any liquid tends to seek itsown level. This means that the brake fluid in the master cylin-der tends to flow downward toward the wheel cylinders orcalipers. As long as the brake fluid level in the master cylinderis higher than the bleeder valve, the brake fluid will flow down-ward and out the open bleeder valve, as shown in Figure 8-8.

This flow of brake fluid can even get past the meteringvalve and proportioning valve. The proportioning valve is nor-mally open to the rear brakes until the pressure reaches a pre-determined level when it starts to limit increasing pressure tothe rear brakes. The metering valve used to control or delaythe operation of the front brakes is open to the front wheelsuntil the pressure exceeds 10 to 15 psi (70 to 100 kPa). There-fore, as long as no one is pushing on the brake pedal, the me-tering valve remains open to the front wheels and the brakefluid from the master cylinder can easily flow downwardthrough the valve and out the open bleeder valve.

Since no pressure is exerted on the brake fluid, the largeair bubbles remain large air bubbles and are not separated intosmaller, harder-to-bleed air bubbles that can occur with man-ual bleeding.

All four wheel brakes can be bled at one time using thegravity method. In this process, the bleeder screws at all fourwheels are opened at the same time, and the system is allowed

136 CHAPTER 8

TechTipTHE MASTER

CYLINDER ONE-

DRIP-PER-SECOND

TEST

Excessive brake wear is often caused by misadjusted brakelinkage or brake light switches keeping the brake pedalfrom fully releasing. If the brake pedal is not fully released,the primary piston sealing cup blocks the compensatingport from the brake fluid reservoir. To test if this is theproblem, loosen both lines from the master cylinder. Brakefluid should drip out of both lines about one drip per sec-ond. This is why this test is also called the “Master Cylin-der Drip Test.” If the master cylinder does not drip, thebrake pedal may not be allowing the master cylinder tofully release. Have an assistant pull up on the brake pedal.If the dripping starts, the problem is due to a misadjustedbrake light or speed (cruise) control switch or pedal stop.If the master cylinder still does not drip, loosen the mastercylinder from the power booster. If the master cylindernow starts to drip, the pushrod adjustment is too long.

If the master cylinder still does not drip, the prob-lem is in the master cylinder itself. Check for brake fluidcontamination. If mineral oil, such as engine oil, powersteering fluid, or automatic transmission fluid (ATF), hasbeen used in the system, the rubber sealing cups swelland can block off the compensating port. If contamina-tion is discovered, every brake component that containsrubber must be replaced.

to drain naturally until the fluid coming out of the bleeders isfree of air.

Gravity bleeding is a slow process that can take an houror more. In addition, this procedure cannot be used onbrake systems with residual pressure check valves becausethe valves restrict the fluid flow. The advantage of gravitybleeding is that it can be done by a single technician, whois freed to attend to other jobs while the brakes bleed. Whenother bleeding procedures fail, gravity bleeding can some-times be effective on brake systems that trap small pocketsof air.

Gravity bleeding requires a bleeder wrench, four lengthsof plastic hose that fit snugly over the bleeder screws, and fourjars to catch the dripping fluid. Unless a plastic hose is used to“start a siphon” at each bleeder screw, it is possible that airmay enter the system rather than be bled from it. This can oc-cur because the total open area of the four bleeder screws issomewhat larger than that of the two compensating portsthrough which the fluid must enter the system. To gravitybleed the brake system, follow these steps.

1. Fill the master cylinder reservoir with new brake fluid.During the bleeding process, check the fluid level peri-odically to ensure that the reservoir remains at leasthalf full.

2. Attach a length of plastic tubing to each bleeder screw, andplace the ends of the tubes in jars to catch the drainage.

3. Open each bleeder screw approximately one full turnand make sure that fluid begins to drain. Allow the sys-tem to drain until the fluid flowing from the bleederscrews is free of air bubbles.

4. Close the bleeder screws and top up the fluid level in themaster cylinder reservoir.

FIGURE 8-8 Gravity bleeding issimply opening the bleeder valve and al-lowing gravity to force the brake,fluid out ofthe bleeder valve.Because air is lighter thanbrake fluid all of the air escapes before thebrake fluid runs out.


TechTipGRAVITY BLEED

DURING AN OIL

CHANGE

Brake fluid tends to absorb moisture near the wheelbrakes through the flexible brake lines and other con-nections. Some service technicians open the bleedervalves whenever the vehicle is in for an oil change serv-ice. As the brake fluid slowly flows from the openbleeder valve, any trapped air is also released. After afew minutes, the bleeder valves are closed and the ve-hicle lowered. The brake fluid level is checked and theoil change is completed. This procedure not only helpsprovide the vehicle owner with an air-free brake system,but opening the bleeder at every oil change helps keepthe bleeder free and easy to open.

FLUID CHAMBER

AIR CHAMBER

FIGURE 8-9 A typical pressure bleeder.The brake fluid inside is pres-surized with air pressure in the air chamber. This air pressure is applied to thebrake fluid in the upper section. A rubber diaphragm separates the air from thebrake fluid. (Courtesy of EIS Brake Parts)

PRESSURE BLEEDING

Pressure bleeding, sometimes called power bleeding, is acommon method used to bleed the brake hydraulic system. Inthis process, a pressure bleeder attached to the master cylinderforces brake fluid through the system under pressure to purgeany trapped air. Once the hydraulic system is pressurized, thetechnician simply opens the bleeder screws in the prescribedorder and allows fluid to flow until it is free of air bubbles.

The tools required for pressure bleeding include a plastichose and fluid catch jar as used in manual bleeding, as well asa pressure bleeder, a source of air pressure to charge thebleeder, and an adapter to attach the pressure bleeder to themaster cylinder fluid reservoir. Cast-metal cylinders with inte-gral reservoirs commonly use a flat, plate-type adapter thatseals against the same surface as the reservoir cover. SeeFigure 8-9.

Some plastic master cylinder reservoirs also use plate-typeadapters, but others require adapters that seal against the bot-tom of the reservoir. Pressure bleeder manufacturers offermany adapters to fit specific applications. See Figure 8-10.

METERING VALVE

OVERRIDE TOOLS

In addition to the tools previously described, a metering valveoverride tool is required when pressure bleeding the frontbrakes of certain vehicles. The override tool is used to deacti-

vate the metering valve because the operating pressure ofpower bleeders is within the range where the metering valveblocks fluid flow to the front brakes. Metering valves that re-quire an override tool have a stem or button on one end thatis either pushed in or pulled out to hold the valve open. Theoverride tool performs this service.

To install the override tool used on General Motors vehi-cles, loosen the combination valve mounting bolt and slip theslot in the tool under the bolt head. See Figure 8-11.

Push the end of the tool toward the valve body until it de-presses the valve plunger, then tighten the mounting bolt tohold the tool in place.

138 CHAPTER 8

FIGURE 8-10 Brake fluid under pressure from the power bleeder isapplied to the top of the master cylinder. It is very important that the properadapter be used for the master cylinder.Failure to use the correct adapter or fail-ure to release the pressure on the brake fluid before removing the adapter cancause brake fluid to escape under pressure.

VALVESTEM

OVERRIDETOOL

COMBINATIONVALVE

FIGURE 8-11 Metering valve override tool on a General Motorsvehicle.

Some full-size Ford vehicles have a metering valve with astem that must be pushed in to bleed the front brakes, but Forddoes not offer a special tool for this purpose. An assistant is neededto override the valve when the front brakes are being bled.

To install the override tool used on older Chrysler andFord vehicles, slip one fork of the tool under the rubberboot, and the other fork under the valve stem head. SeeFigure 8-12.

The spring tension of the tool holds the valve open, but al-lows the valve stem to move slightly when the system is pressur-ized. If the valve is held rigidly open, internal damage will result.

PRESSURE BLEEDING

PROCEDURE

Just as in manual bleeding, it is important to follow the propersequence when pressure bleeding a brake system. Some man-ufacturers recommend one sequence for manual bleeding andanother for pressure bleeding. To pressure bleed a brake sys-tem, follow these steps.

1. If it has not already been done, consult the equipmentmanufacturer’s instructions and fill the pressure bleederwith the proper type of brake fluid.

2. Make sure the bleeder is properly sealed and the fluidsupply valve is closed, then use compressed air to pres-surize the bleeder until approximately 30 psi (207 kPa)is indicated on the bleeder gauge.

3. If the vehicle is equipped with a metering valve, overrideit with the appropriate tool.

METERINGVALVE

COMBINATIONVALVE

VALVESTEM

OVERRIDETOOL

OVERRIDE TOOL

VALVESTEM

CHRYSLER

FORD

FIGURE 8-12 Pull-out-type metering valves being held out using aspecial override tool.


FIGURE 8-13 A reverse fluid injection unit that is used to bleed hy-draulic brake and clutch systems by forcing brake fluid up from the bleeder valveinto the master cylinder,thereby forcing any trapped air up and out of the system.

FrequentlyAsked Question

WHAT IS REVERSE FLUIDINJECTION?

Reverse fluid injection is a procedure that uses an air-or hand-operated injection gun that pushes brake fluidfrom the bleeder valve into the hydraulic system. SeeFigure 8-13.

By forcing brake fluid into the bleeder valve, anytrapped air is forced upward into the master cylinder.

CAUTION: This procedure should only be done after a thoroughflushing of the hydraulic system. Many experts warn that debris andsediment in the hydraulic system can be back-flushed into the ABS hy-draulic unit and/or master cylinder. Many brake and ABS failures havebeen caused by forcing old brake fluid back into the system.

4. Clean the top of the master cylinder, then remove the mas-ter cylinder cover and clean around the gasket surface. Becareful not to allow any dirt to fall into the reservoir.

5. Fill the reservoir about half full with new brake fluid,then install the proper pressure bleeder adapter on themaster cylinder.

6. Connect the pressure bleeder fluid supply hose to theadapter, making sure the hose fitting is securely engaged.

7. Open the fluid supply valve on the pressure bleeder to al-low pressurized brake fluid to enter the system. Checkcarefully for fluid leaks that can damage the vehicle finish.

8. Slip the plastic hose over the bleeder screw of the firstwheel cylinder or caliper to be bled, and submerge theend of the tube in the jar of brake fluid.

9. Open the bleeder screw approximately one-half turn,and let the fluid run until air bubbles no longer emergefrom the tube. Close the bleeder screw.

10. Transfer the plastic hose to the bleeder screw of the nextwheel cylinder or caliper in the bleeding sequence, andrepeat steps 9 and 10. Continue around the vehicle in thespecified order until the brakes at all four wheels havebeen bled.

11. Remove the metering valve override tool.12. Close the fluid supply valve on the pressure bleeder.13. Wrap the end of the fluid supply hose in a shop towel,

and disconnect it from the master cylinder adapter. Donot spill any brake fluid on the vehicle finish.

14. Remove the master cylinder adapter, adjust the fluidlevel to the full point, and install the fluid reservoir cover.

SURGE BLEEDING

Surge bleeding is a supplemental bleeding method used to helpremove air bubbles that resist other bleeding processes. In surgebleeding, the brake pedal is pumped rapidly to create turbulencein the hydraulic system. This agitation helps dislodge air bubblesthat cling to the pores of rough castings, or become trapped athigh points or turns in the brake lines. Surge bleeding is not rec-ommended for systems filled with silicone DOT 5 brake fluids.These fluids tend to trap tiny air bubbles that are very difficult tobleed from the hydraulic system. The added agitation of surgebleeding only makes the problem worse.

Surge bleeding requires an assistant to pump the brakepedal, a bleeder screw wrench, approximately two feet of clear,plastic hose with an inside diameter small enough to fit snuglyover the bleeder screw, and a jar partially filled with clean brakefluid. To surge bleed a brake system, follow these steps.

1. Slip the plastic hose over the bleeder screw of the wheelcylinder or caliper to be bled and submerge the end ofthe tube in the jar of brake fluid.

2. Open the bleeder screw approximately one-half turn.3. With the bleeder screw open, have your assistant rapidly

pump the brake pedal several times. Air bubbles shouldcome out with the brake fluid.

4. While your assistant holds the brake pedal to the floor,close the bleeder screw.

5. Repeat steps 2 through 4 at each bleeder screw in therecommended order.

6. Re-bleed the system using one of the four other methodspreviously described.

140 CHAPTER 8

FIGURE 8-14 Note the large drops being squirted from the top of themaster cylinder due to trapped air when the brakes were released.

BINDER CLIPFENDER COVER

FIGURE 8-15 It is very important to use fender covers to protect thepaint of the vehicle from being splashed with brake fluid.Use a binder clip avail-able at local office supply stores to clip the fender cover to the lip of the fender,thereby preventing the fender cover from slipping.

FLUID CHANGING

In addition to removing air, brake systems need to be bledin order to clean out old and/or contaminated fluid. Thisprocess is called fluid changing or flushing. The hygroscopic(water attracting) nature of most brake fluid makes it nec-essary to flush the brake system periodically because mois-

ture in the fluid drastically lowers the boiling point. Wateralso causes rust and corrosion of system components thatcan lead to brake failure. To avoid these problems, brakefluid should be changed yearly in humid climates and everyother year in dry climates.

TechTipQUICK AND EASY

TEST FOR AIR IN

THE LINES

If air is in the brake lines, the brake pedal will be lowand will usually feel “spongy” or “mushy.” To confirmthat trapped air in the hydraulic system is the cause, per-form this simple and fast test:

Step 1 Remove the cover from the master cylinder.Have an assistant pump the brake pedal sev-eral times and then hold the pedal down.

Step 2 Observe the squirts of brake fluid fromthe master cylinder when the brake pedalis quickly released. (This is best per-formed by allowing the foot to slip off theend of the brake pedal.)

ResultsIf the brake fluid squirts higher than 3 in. (8 cm) fromthe surface, then air is trapped in the hydraulic system.See Figure 8-14.

CAUTION: Always use a fender cover whenever performing this test.Brake fluid will remove paint if it gets onto the unprotected fender. SeeFigure 8-15 for an example of how to keep fender covers from slipping off.

ExplanationAir can be compressed; however, liquid cannot be com-pressed. When pumping the brake pedal, the assistant iscompressing any trapped air. When the pedal is releasedquickly, the compressed air expands and takes up morevolume, forcing the brake fluid upward through thecompensation ports into the reservoir. Some upwardmovement is normal because of the return spring pres-sure on the valves in the master cylinder and springs inthe wheel cylinders. If, however, the spurt is higher thannormal, this is a sure sign of air being trapped in the sys-tem. This test is also called “the air entrapment test.”


(a)

(b)

FIGURE 8-16 (a) A turkey baster can be used to remove the old brakefluid from the master cylinder reservoir.(b) Use care to avoid dripping brake fluidafter draining it from the master cylinder reservoir.

Brake fluid changing or flushing is done by bleeding thesystem until all of the old fluid is purged from the system.Because fluid changing requires the ability to flush out con-tamination and move a great deal of fluid, the best methodto use is pressure bleeding. Other acceptable choices aremanual bleeding and vacuum bleeding. Gravity bleeding isnot recommended for fluid changing because it is slow, andwithout significant pressure behind the fluid flow, there isno guarantee that contamination will be completely flushedfrom the system.

To change a vehicle’s brake fluid, remove all the old brakefluid from the master cylinder reservoir.

HINT: A low-cost turkey baster can be used to draw the old brake fluid from

the master cylinder reservoir. See Figure 8-16.

Fill the reservoir with new fluid, then follow the proce-dures outlined earlier in the chapter for the chosen method ofbleeding. Continue to bleed at each wheel until the fluid thatemerges from the bleeder screw is free of any discolorationand contamination.

142 CHAPTER 8

Step 6 A vacuum-type bleeder being used to draw brake fluidfrom the open bleeder valve into the hand-held unit.Shop air is used to create the vacuum to draw the fluidfrom the caliper.

Step 4 Use a hammer to tap the wrench to the counter-clock-wise position.This should loosen the bleeder valve.

Step 3 Place a 6-point box-end wrench onto the bleeder valveand use a hammer to lightly tap the wrench clockwiseas if to tighten it.The shock of the hammer blow againhelps loosen the bleeder valve.

Step 5 The tapered portion of the bleeder valve is what sealsthe brake fluid in the caliper as shown. The repeatedshock blows help break this taper. If the taper is notbroken, the bleeder valve is often broken, requiringthat the old bleeder valve be drilled and the caliper betapped for an oversize bleeder valve.

Step 1 The first step in brake bleeding is to remove the protec-tive rubber cap that covers the bleeder valve.

Step 2 To help loosen the bleeder valve, use a hammer to tapon the end of it.The shock of the hammer blow helpsfree the bleeder valve.

Brake Bleeding Step-by-Step

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Step 9 Tighten the bleeder valve.

Brake Bleeding Step-by-Step

Step 7 Another method that can be used to bleed a caliper iscalled a manual method. To manually bleed a caliper,loosen the bleeder valve about one full turn.

Step 8 To prevent the possibility of harming the master cylin-der, place a block of wood under the brake pedal.Thisblock limits the brake pedal to normal travel.When youdepress the brake pedal,air and some fluid should flowfrom the bleeder valve.

Step 10 After tightening the bleeder valve, release the brakepedal. Failure to tighten the bleeder before releasingthe brake pedal will cause air to be drawn back into thecaliper.Repeat the procedure as needed until only fluidis observed flowing from the bleeder valve.

Step 12 With the brake pedal depressed,the primary seals insidethe master cylinder block the fluid in the reservoir fromdripping out of an open line.Note in this picture that thebleeder valve has been completely removed from thecaliper and the brake fluid is not running out.

Step 11 A commom trick that can be used to reduce the needto bleed the system after a component has been re-placed is to use a brake pedal depressor to depress thebrake pedal about 1 in (25 mm).

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144 CHAPTER 8

Summary

1. Bleeding the brakes means to pump the hydraulic systemfree of air.

2. A new or replacement master cylinder should be bled be-fore installing it in the vehicle.

3. Bleeder valves are located on all disc brake calipers anddrum brake wheel cylinders.

4. The most commonly used method of brake bleeding isthe single stroke bleeding method.

5. Vacuum bleeding is used to draw the old fluid and anytrapped air from the hydraulic system through thebleeder valve.

6. Gravity bleeding is an excellent but slow method.7. Pressure bleeding requires adapters and special equip-

ment, plus the metering valve must be held open to allowfluid to flow to the front brakes.

8. Surge bleeding is the least desirable method of bleeding,but is necessary at times.

Review Questions

1. Describe how to bench bleed a master cylinder.

2. List the steps necessary to manually bleed a brake hydraulicsystem.

3. Discuss the equipment and the procedures needed to pressurebleed a brake hydraulic system.

4. Explain how to gravity bleed a brake hydraulic system.

5. Describe how to vacuum bleed a brake hydraulic system.

6. Discuss the surge method of brake bleeding.

Chapter Quiz

1. The button on the ___________ valve should be held whenbleeding the brakes.

a. Metering

b. Proportioning

c. Pressure-differential

d. Residual check

2. The brake bleeding procedure usually specified for a rear-wheelvehicle with a dual split master cylinder is ___________.

a. RR, LR, RF, LF

b. LF, RF, LR, RR

c. RF, LR, LF, RR

d. LR, RR, LF, RF

3. Two technicians are discussing bench bleeding a master cylin-der. Technician A says that the front (nose end) of the mastercylinder should be bled first. Technician B says that rear (brakepedal end) should be bled first. Which technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

4. Two technicians are discussing how to loosen bleeder valves.Technician A says that a shock is usually necessary to break thetaper at the base of the valve. Technician B says to apply steady,loosening torque to the bleeder valve using a 6-point wrench.Which technician is correct?





5. Technician A says that the brake pedal should be depressedwith as much force as possible during the normal bleeding pro-cedure. Technician B says that the brake pedal should bepumped rapidly during this manual bleeding procedure toforce the air down toward the bleeder valve(s). Which techni-cian is correct?






6. Vacuum is applied where during vacuum bleeding?

a. At each wheel brake bleeder valve

b. At the master cylinder reservoir

c. At the vacuum booster check valve

d. At the metering valve end of the combination valve

7. The bleeder is opened at a caliper and no brake fluid flows out.Technician A says that this is normal and that brake fluid shouldnot flow or drip out of an open bleeder valve. Technician B saysthat the vent port in the master cylinder may be blocked. Whichtechnician is correct?





8. The usual maximum pressure that should be used when pres-sure bleeding a brake system is ___________.

a. 20 psi

b. 30 psi

c. 50 psi

d. 70 psi

9. Why do many vehicle manufacturers recommend that brakefluid be changed regularly?

a. Brake fluid absorbs moisture over time

b. The boiling point of the brake fluid decreases

c. Old brake fluid can cause rust and corrosion of brake systemcomponents

d. All of the above

10. Air in the brake hydraulic system can cause all except___________.

a. Spongy brake pedal

b. Hard brake pedal

c. Lower than normal brake pedal

d. May require the driver to “pump up” the brakes before theywill stop the vehicle

Documents

Brakes Ch08