Brake bleeding is removing any trapped air from the hydraulic system. Air can get into the hydraulic system whenever any hydraulic brake line or unit is opened. A common source of air in the brake system of this type can occur through very small holes in rubber flexible brake lines. Another source is absorption of moisture by brake fluid. When moisture is absorbed, boiling point of the brake fluid is reduced. During severe braking, the heat generated can cause the brake fluid to 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. Air in the system results in a spongy brake pedal.
BLEEDING THE MASTER CYLINDER
Whenever the master cylinder is replaced or the hydraulicsystem has been left opened for several hours, the air may have to be bled from the master cylinder. Bleed the master cylinder “on the bench” before installing it on the vehicle.
Continued Figure 72–1 Always clamp a master cylinder in a vise by the mounting flange to prevent distortion of the cylinder bore.
If bleeding the master cylinder after working on the hydraulic system, follow these steps:
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.
Attempting to loosen a bleeder valve often results in breaking (shearing off) the bleeder valve. Several procedures can be tried that help prevent the possibility of breaking a bleeder valve. Bleeder valves are tapered and become wedged in the caliper on the wheel cylinder housing.
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
Air Impact Method Use a 6-point socket for the bleeder valve and necessary adapters to fit an air impact wrench to the socket. Apply some penetrating oil to the bleeder valve and allow it to flow around the threads. Turn the pressure down on the impact wrench to limit the force. The hammering effect of the impact wrench loosens the bleeder valve without breaking it off.
Hit and Tap Method
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.
Air Punch Method Use an air punch near the bleeder valve while attempting to loosen the bleeder valve at the same time.
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.
Heat and Tap Method Heat the area around the bleeder valve with a torch. The heat expands the size of the hole and usually allows the bleeder to be loosened and removed.
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.
Step #1 Heat the bleeder valve itself with a torch. The heat causes the valve itself to expand. Step #2 Remove heat from the bleeder valve. As the valve is cooling, touch paraffin wax or candle wax to the hot valve. The wax will melt and run down around the threads of the valve. Step #3 Allow the bleeder valve to cool until it can be safely touched with your hand. This assures that the temperature is low enough for the wax to return to a solid and provide the lubricating properties necessary for the easy removal of the bleeder valve. Again, turn the bleeder valve clockwise before turning the valve counterclockwise to remove.
Continued Wax Method
Often, used brake fluid looks like black coffee or coffee with cream. Both conditions indicate contaminated or moisture-laden brake fluid that should be replaced. The following steps will assure a complete brake fluid change:
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.
After bleeding the master cylinder, the combination valve should be bled if equipped. Follow the same procedure as when bleeding the master cylinder, being careful not to allow the master cylinder to run dry.
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.
Check the level in the master cylinder frequently and keep it filled with clean brake fluid throughout the brake bleeding procedure.
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:
For vehicles equipped with a diagonal split section or equipped with ABS, follow the brake bleeding procedure recommended in the service information for the vehicle.
NOTE: If the vehicle has two wheel cylinders on one brake, bleed the upper cylinder first.
Manual bleeding uses hydraulic pressure created by the master cylinder to pump fresh fluid through the brake system. This method is called the single stroke bleeding method .
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.
To manually bleed the brake system, follow these steps:
Discharge the vacuum or hydraulic power booster (if equipped) by pumping the brake pedal with the ignition OFF until the pedal feels hard.
Fill the master cylinder reservoir with new brake fluid and make sure it remains at least half full throughout the bleeding procedure.
Attach the plastic hose over the bleeder screw of the first wheel cylinder or caliper in the bleeding sequence, and submerge the end of the tube in the jar of brake fluid. See Figure 72–6.
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.
Loosen the bleeder screw approximately one-half turn, and have an assistant slowly depress the brake pedal. Air bubbles leaving the bleeder screw will be visible in the hose.
Tighten the bleeder screw and have your assistant slowly release the brake pedal.
Wait at least 15 seconds to allow time for any small bubbles to form into larger bubbles.
Repeat steps 4 and 5 until no more air bubbles emerge from the bleeder.
Transfer the plastic hose to the bleeder screw of the next wheel cylinder or caliper in the bleeding sequence, and repeat steps 4 through 7. Continue until all brakes are bled.
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 uses a special suction pump that attaches to the bleeder screw. The pump creates a low-pressure area at the bleeder screw, which allows atmospheric pressure to force brake fluid through the system when the bleeder screw is opened.
Continued Vacuum bleeding requires only one technician. To vacuum bleed a brake system follow these steps:
Fill the master cylinder reservoir with new brake fluid and make sure it remains at least half full throughout the bleeding procedure.
Attach the plastic tube from the vacuum bleeder to the bleeder screw of the first wheel cylinder or caliper in the bleeding sequence.
Figure 72–7 Vacuum bleeding uses atmospheric pressure to force brake fluid through the hydraulic system.
If necessary, use one of the adapters provided with the vacuum in the catch bottle.
Operate the pump handle to create a partial vacuum in the catch bottle.
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.
Repeat steps 3 and 4 until no more air bubbles emerge from the bleeder.
Do not use excessive brake pedal force while bleeding and never bleed the brake with the engine running! The extra assist from the power brake unit greatly increases the force exerted on the brake fluid in the master cylinder. The trapped air bubbles may be dispersed into tiny bubbles that often cling to the inside surface of the brake lines.
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 is a slow, effective, method that works on many vehicles to rid the hydraulic system of air. The bleeder valve is opened until brake fluid flows from the open valve.
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 .
Gravity bleeding works because any liquid tends to seek its own level. This means that the brake fluid in the master cylinder tends to flow downward toward the wheel cylinders or calipers. As long as the brake fluid level in the master cylinder is higher than the bleeder valve, brake fluid will flow downward and out the bleeder valve.
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
Excessive brake wear is often caused by misadjusted brake linkage or brake light switches keeping the brake pedal from fully releasing. If the brake pedal is not fully released, the primary piston sealing cup blocks the compensating port from the brake fluid reservoir.
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.
All four wheel brakes can be bled at one time using the gravity method. In this process, the bleeder screws at all four wheels are opened at the same time, and the system is allowed to drain naturally until the fluid coming out of the bleeders is free of air.
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.
Gravity bleeding requires a bleeder wrench, four lengths of plastic hose that fit snugly over the bleeder screws, and four jars to catch the dripping fluid.
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.
To gravity bleed the brake system, follow these steps:
Fill the master cylinder reservoir with new brake fluid. During bleeding, check the fluid level periodically to ensure that the reservoir remains at least half full.
Attach a length of plastic tubing to each bleeder screw, place the ends of the tubes in jars to catch drainage.
Open each bleeder screw approximately one full turn and make sure that fluid begins to drain. Allow the system to drain until the fluid flowing from the bleeder screws is free of air bubbles.
Close the bleeder screws and top up the fluid level in the master cylinder reservoir.
Brake fluid tends to absorb moisture near the wheel brakes through, the flexible brake lines and other connections. Some service technicians open the bleeder valves whenever the vehicle is in for an oil change service. As the brake fluid slowly flows from the open bleeder valve, any trapped air is also released. After a few minutes, the bleeder valves are closed and the vehicle lowered. The brake fluid level is checked and the oil change is completed. This procedure not only helps provide the vehicle owner with an air-free brake system, but opening the bleeder at every oil change helps keep the bleeder free and easy to open.
Gravity Bleed During an Oil Change
Pressure bleeding , called power bleeding , is a common method used to bleed the brake hydraulic system. A pressure bleeder attached to the master cylinder forces brake fluid through the system under pressure to purge any trapped air.
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)
Cast-metal cylinders with integral reservoirs use a flat, plate-type adapter that seals against the same surface as the reservoir cover.
Some plastic master cylinder reservoirs also use plate-type adapters, but others require adapters that seal against the bottom of the reservoir. Pressure bleeder manufacturers offer many adapters to fit specific applications.
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
A metering valve override tool is required when pressure bleeding the front brakes of certain vehicles.
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
Some full-size Ford vehicles have a metering valve with a stem that must be pushed in to bleed the front brakes, but Ford does not offer a special tool for this purpose. An assistant is needed to override the valve when the front brakes are being bled.
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
Just as in manual bleeding, it is important to follow the proper sequence when pressure bleeding a brake system. Some manufacturers recommend one sequence for manual bleeding and another for pressure bleeding.
Continued To pressure bleed a brake system, follow these steps:
Consult the equipment manufacturer’s instructions and fill the pressure bleeder with the proper type of brake fluid.
Make sure the bleeder is properly sealed and the fluid supply valve is closed, then use compressed air to pressurize the bleeder until approximately 30 psi (207 kPa) is indicated on the bleeder gauge.
If the vehicle is equipped with a metering valve, override it with the appropriate tool.
Clean the top of the master cylinder, then remove the master cylinder cover and clean around the gasket surface. Be careful not to allow any dirt to fall into the reservoir.
Fill the reservoir about half full with new brake fluid, install the proper pressure bleeder adapter on the master cylinder.
Connect the pressure bleeder fluid supply hose to the adapter, making sure the hose fitting is securely engaged.
Open the fluid supply valve on the pressure bleeder to allow pressurized brake fluid to enter the system. Check carefully for fluid leaks that can damage the vehicle finish.
Slip the plastic hose over the bleeder screw of the first wheel cylinder or caliper to be bled, and submerge the end of the tube in the jar of brake fluid.
Open bleeder screw approximately one-half turn, let fluid run until air bubbles no longer emerge. Close the bleeder screw.
Transfer the plastic hose to the bleeder screw of the next wheel cylinder or caliper in the bleeding sequence, and repeat steps 8 and 9. Continue around the vehicle until the brakes at all four wheels have been bled.
Remove the metering valve override tool.
Close the fluid supply valve on the pressure bleeder.
Wrap the end of the fluid supply hose in a shop towel, and disconnect it from the master cylinder adapter. Do not spill any brake fluid on the vehicle finish.
Remove the master cylinder adapter, adjust the fluid level to the full point, and install the fluid reservoir cover.
Surge bleeding is a supplemental bleeding method used to help remove air bubbles that resist other bleeding processes. In surge bleeding, the brake pedal is pumped rapidly to create turbulence in the hydraulic system. Surge bleeding is not recommended for systems filled with silicone DOT 5 brake fluids. These fluids tend to trap tiny air bubbles that are very difficult to bleed from the hydraulic system. Added agitation of surge bleeding only makes the problem worse. Surge bleeding requires an assistant to pump the brake pedal, a bleeder screw wrench, two feet of clear, plastic hose with an inside diameter small enough to fit snugly over the bleeder screw, and a jar partially filled with clean brake fluid.
To surge bleed a brake system, follow these steps:
Slip the plastic hose over the bleeder screw of the wheel cylinder or caliper to be bled and submerge the end of the tube in the jar of brake fluid.
Open the bleeder screw approximately one-half turn.
With the bleeder screw open , have your assistant rapidly pump the brake pedal several times. Air bubbles should come out with the brake fluid.
While your assistant holds the brake pedal to the floor, close the bleeder screw.
Repeat steps 2 through 4 at each bleeder screw in the recom mended order.
Re-bleed the system using one of the four other methods previously described.
Reverse fluid injection is a procedure that uses air-or hand-operated injection gun that pushes brake fluid from the bleeder value into the hydraulic system. By forcing brake fluid into the bleeder valve, any trapped air is forced upward into the master cylinder.
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.
In addition to removing air, brake systems need to be bled in order to clean out old and/or contaminated fluid. This process is called fluid changing or flushing. Brake fluid changing or flushing is done by bleeding the system until all of the old fluid is purged from the system. Because fluid changing requires the ability to flush out contamination and move a great deal of fluid, the best method to use is pressure bleeding. Gravity bleeding is not recommended for fluid changing because it is slow, and without significant pressure behind the fluid flow, there is no guarantee that contamination will be completely flushed from the system.
To change a vehicle’s brake fluid, remove all the old brake fluid from the master cylinder reservoir. Fill the reservoir with new fluid, then follow the procedures outlined earlier in the chapter for the chosen method of bleeding. Continue to bleed at each wheel until the fluid that emerges from the bleeder screw is free of any discoloration and contamination.
Bleeding the brakes means to pump the hydraulic system free of air.
A new or replacement master cylinder should be bled before installing it in the vehicle.
Bleeder valves are located on all disc brake calipers and drum brake wheel cylinders.
The most commonly used method of brake bleeding is the single stroke bleeding method.
Vacuum bleeding is used to draw the old fluid and any trapped air from the hydraulic system through the bleeder value.
Gravity bleeding is an excellent but slow method.
Pressure bleeding requires adapters and special equipment, plus the metering valve must be held open to allow fluid to flow to the front brakes.
Surge bleeding is the least desirable method of bleeding, but is necessary at times.