Chap72

[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 72, the reader should be able to: Continued

[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 72, the reader should be able to:

BRAKE BLEEDING ,[object Object],Continued

BLEEDING THE MASTER CYLINDER ,[object Object],Continued Figure 72–1 Always clamp a master cylinder in a vise by the mounting flange to prevent distortion of the cylinder bore.

[object Object],Continued Step #1 Fill the master cylinder with clean brake fluid from a sealed container up to the recommended “full” level. Step #2 Have an assistant slowly depress the brake pedal as you “crack open” the master cylinder bleed screw starting with the section closest to the bulkhead. It is very important that the primary section of the master cylinder be bled before attempting to bleed the air out of the secondary section of the master cylinder. Before the brake pedal reaches the floor, close the bleeder valve. Step #3 Repeat several times until a solid flow of brake fluid is observed leaving the bleeder valve. If the master cylinder has no bleeder valves, outlet tube nuts can be loosened instead.

NOTE: A proper manual bleeding of the hydraulic system requires that accurate communications occur between the person depressing the brake pedal and the person opening and closing the bleeder valve(s). The bleeder valve (also called a bleed valve ) should be open only when the brake pedal is being depressed. The valve must be closed when the brake pedal is released to prevent air from being drawn into the system.

[object Object],BRAKE BLEEDER VALVE LOOSENING METHODS Continued Figure 72–2 Typical bleeder valve from a disc brake caliper. The arrows point to the taper section that does the actual sealing. It is this taper that requires a shock to loosen. If the bleeder is simply turned with a wrench, the bleeder usually breaks off because the tapered part at the bottom remains adhered to the caliper or wheel cylinder. Once loosened, brake fluid flows around the taper and out through the hole in the side of the bleeder valve.

Figure 72–3 Typical bleeder locations. Note that the combination valve and master cylinder shown do not have bleeder valves; therefore, bleeding is accomplished by loosening the brake line at the outlet parts. (Courtesy of Allied Signal Automotive Aftermarket) Continued

[object Object],Continued Step #1 Tap on the end of the bleeder valve with a steel hammer. This shock often “breaks the taper” at the base of the bleeder valve. Shock also breaks loose rust or corrosion on the threads. Step #2 Using a 6-point wrench or socket, tap the bleeder valve in the clockwise direction (tighten). Step #3 Using the same 6-point socket or wrench, tap the bleeder valve counterclockwise to loosen and remove the bleeder valve. Step #4 If valve is still stuck (frozen), repeat Step 1 thru Step 3. NOTE: The shock of the tap on the wrench breaks loose the valve. Simply pulling on the wrench often results in breaking off the bleeder.

[object Object],Figure 72–4 Using an air punch next to the bleeder valve to help “break the taper” on the bleeder valve. Continued The punch creates a shock motion that often loosens the taper and threads of the bleeder valve from the caliper or wheel cylinder. It is often helpful to first attempt to turn the bleeder valve in the clockwise (tightening), then turn in the counterclockwise direction to loosen and remove the bleeder valve.

[object Object],Continued 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 require that all internal rubber parts be replaced.

[object Object],Continued Wax Method

[object Object],Do It Right — Replace the Brake Fluid Step #1 Remove the old brake fluid from the master cylinder using a suction bulb. (Dispose of this old brake fluid properly.) Step #2 Fill the master cylinder with new clean brake fluid from a sealed container. Step #3 Bleed each wheel brake until the brake fluid is clean. This fluid replacement will fully restore the brake hydraulic system to as-new condition and protect the system from rust and corrosion. Replacing only friction pads and/or linings is not a complete, thorough brake system service. Customers should be aware of the importance of this procedure. CAUTION: Do not allow the master cylinder to run out of brake fluid. Recheck and refill as necessary during the bleeding process.

BLEEDING SEQUENCE ,[object Object],Continued 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.

[object Object],Figure 72–5 Most vehicle manufacturers recommend starting the brake bleeding process at the rear wheel farthest from the master cylinder. Continued For most rear-wheel-drive vehicles equipped with a front/rear split system, start the bleeding with the wheel farthest from the master cylinder and work toward the closest. For most vehicles, this sequence is: ,[object Object],[object Object],[object Object],[object Object]

[object Object],NOTE: If the vehicle has two wheel cylinders on one brake, bleed the upper cylinder first.

MANUAL BLEEDING ,[object Object],Continued It is extremely important when manually bleeding a brake system that the pedal be applied and released slowly and gently. Rapid pedal pumping can churn up the fluid and reduce the size of trapped air bubbles, making them more difficult to bleed. Manual bleeding requires an assistant to apply and release the brake pedal, a bleeder screw wrench, approximately two feet of clear, plastic hose with an inside diameter small enough to fit snugly over the bleeder screws, and a clear jar partially filled with clean brake fluid.

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Figure 72–6 Bleeding brakes using clear plastic tubing makes it easy to see air bubbles. Submerging the hose in a container of clean brake fluid helps ensure that all of the air will be purged by the system. ,[object Object],[object Object],Continued

[object Object],[object Object],[object Object],NOTE: Make certain all brake components such as calipers and wheel cylinders are correctly installed with bleeder valve located on the highest 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 ,[object Object],Continued Vacuum bleeding requires only one technician. To vacuum bleed a brake system follow these steps: ,[object Object],[object Object]

Figure 72–7 Vacuum bleeding uses atmospheric pressure to force brake fluid through the hydraulic system. ,[object Object],Continued

Continued ,[object Object],[object Object],[object Object]

[object Object],Tiny Bubbles These tiny air bubbles may not be bled from the hydraulic system until enough time has allowed the bubbles to reform. To help prevent excessive force, do not start the engine. Without power assistance, the brake 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 plastic hammer. After this tapping, simply waiting for a period of time will cause the bubbles to re-form into larger and easier-to-bleed air pockets. Most brake experts recommend waiting 15 seconds or longer between attempts to bleed each wheel. This waiting period is critical and allows time for the air bubbles to form.

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

GRAVITY BLEEDING ,[object Object],Continued Any air trapped in the part being bled will rise and escape from the port when the valve is opened. It may take several minutes before brake fluid escapes. If no brake fluid comes out, remove the bleeder valve entirely—it may be clogged. Nothing but air and brake fluid will be slowly coming out of the wheel cylinder or caliper when the bleeder valve is removed. Do not press on the brake pedal with the bleeder valve out while gravity bleeding .

[object Object],Figure 72–8 Gravity bleeding is simply opening the bleeder valve and allowing gravity to force the brake fluid out of the bleeder valve. Because air is lighter than brake fluid all of the air escapes before the brake fluid runs out. Continued

[object Object],The Master Cylinder 1- Drip Per Second Test - Part 1 To test if this is the problem, loosen both lines from the master cylinder. Brake fluid should drip out of both lines about one drip per second. This is why this test is also called the “Master Cylinder Drip Test” If the master cylinder does not drip, the brake pedal may not be allowing the master cylinder to fully release. Have an assistant pull up on the brake pedal. If the dripping starts, the problem is due to a misadjusted brake light or speed (cruise) control switch or pedal stop, If the master cylinder still does not drip, loosen the master cylinder from the power booster. If the master cylinder now starts to drip, the pushrod adjustment is too long. If the master cylinder still does not drip, the problem is in the master cylinder itself.

[object Object],1- Drip Per Second Test - Part 2 Check for brake fluid contamination. If mineral oil, such as engine oil, power steering fluid, or automatic transmission fluid (ATF), has been used in the system, the rubber sealing cups swell and can block off the compensating port. If contamination is discovered, every brake component that contains rubber must be replaced. Gravity bleeding is a slow process that can take an hour or more. This procedure cannot be used on brake systems with residual pressure check valves because the valves restrict the fluid flow.

[object Object],Continued The advantage of gravity bleeding is that it can be done by a single technician, who is freed to attend to other jobs while the brakes bleed. When other bleeding procedures fail, gravity bleeding can sometimes be effective on brake systems that trap small pockets of air. Unless a plastic hose is used to “start a siphon” at each bleeder screw, it is possible that air may enter the system rather than be bled from it. This can occur because the total open area of the four bleeder screws is somewhat larger than that of the two compensating ports through which the fluid must enter the system.

[object Object],Continued ,[object Object],[object Object],[object Object],[object Object]

[object Object],Gravity Bleed During an Oil Change

PRESSURE BLEEDING ,[object Object],Continued Once the hydraulic system is pressurized, the technician simply opens the bleeder screws in the prescribed order and allows fluid to flow until it is free of air bubbles. Tools required for pressure bleeding include a plastic hose and fluid catch jar as used in manual bleeding, as well as a pressure bleeder, a source of air pressure to charge the bleeder, and adapter to attach the pressure bleeder to the master cylinder reservoir.

Figure 72–9 A typical pressure bleeder. The brake fluid inside is pressurized with air pressure in the air chamber. This air pressure is applied to the brake fluid in the upper section. A rubber diaphragm separates the air from the brake fluid. (Courtesy of EIS Brake Parts) ,[object Object],Continued

[object Object],Figure 72–10 Brake fluid under pressure from the power bleeder is applied to the top of the master cylinder. It is very important that the proper adapter be used for the master cylinder. Failure to use the correct adapter or failure to release the pressure on the brake fluid before removing the adapter can cause fluid to escape under pressure. Continued

METERING VALVE OVERRIDE TOOLS ,[object Object],Continued Metering valves that require an override tool have a stem or button on one end that is either pushed in or pulled out to hold the valve open. The override tool performs this service. The override tool is used to deactivate the metering valve because the operating pressure of power bleeders is within the range where the metering valve blocks fluid flow to the front brakes. To install the override tool used on GM vehicles, loosen the combination valve mounting bolt and slip the slot in the tool under the bolt head. See Figure 72–11.

Figure 72–11 Metering valve override tool on a General Motors Vehicle. Push the end of the tool toward the valve body until it depresses the valve plunger, then tighten the mounting bolt to hold the tool in place. Continued

[object Object],Continued To install the override tool used on older Chrysler and Ford vehicles, slip one fork of the tool under the rubber boot, and the other fork under the valve stem head. The spring tension of the tool holds the valve open, but allows the valve stem to move slightly when the system is pressurized. If the valve is held rigidly open, internal damage will result. See Figure 72–12.

Figure 72–12 Pull-out-type metering valves being held out using a special override tool. Continued

PRESSURE BLEEDING PROCEDURE ,[object Object],Continued To pressure bleed a brake system, follow these steps: ,[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],Continued

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SURGE BLEEDING ,[object Object],Continued

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[object Object],What is Reverse Fluid Injection? Figure 72–13 A reverse fluid injection unit that is used to bleed hydraulic brake and clutch systems by forcing brake fluid up from the bleeder valve into the master cylinder, thereby forcing any trapped air up and out of the system. This procedure should only be done after a thorough hydraulic system flush. Many experts warn that debris and sediment in the hydraulic system can be back-flushed into the ABS hydraulic unit and/or master cylinder. Many brake and ABS failures have been caused by forcing old brake fluid back into the system.

FLUID CHANGING ,[object Object],Continued

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],[object Object],[object Object],[object Object],( cont. )

Chap72

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Chap72