Brake bleeding is the procedure performed on hydraulic brake systems whereby the brake lines (the pipes and hoses containing the brake fluid) are purged of any air bubbles. This is necessary because, while the brake fluid is an incompressible liquid, air bubbles are compressible gas and their presence in the brake system greatly reduces the hydraulic pressure that can be developed within the system.
The process is performed by forcing clean, bubble-free brake fluid through the entire system from the master cylinder(s) to the calipers of disc brakes or the wheel cylinders of drum brakes). The brake bleeder is normally mounted at the highest point on each cylinder or caliper. Brake bleeding can be done easily by two persons working together and doesn't require much technical expertise besides a basic knowledge of brakes and car hydraulics. One wheel at a time, the brake bleeding valve for that wheel is opened while a drain hose is attached to that valve and immersed into a pool of brake fluid. The assistant then pumps the brake pedal. In this manner, fresh fluid is forced through the system into that wheel's caliper or wheel cylinder, and any old, dirty, water-saturated or bubble-entrained fluid is expelled from the system. When the fluid from the caliper or wheel cylinder runs clean, clear, and bubble-free, the brake bleeding valve is closed and the drain tube removed. The process is then repeated for the remaining wheels. During the entire process, the brake fluid reservoir must be kept full of fresh fluid lest new air bubbles be introduced into the brake system.
More sophisticated automatic systems allow brake fluid to be forced under pressure from the brake fluid reservoir into the entire system. Using these devices, no assistant is required.
The term "bleeding the brakes" refers to the process in which a small valve is opened at the caliper (or wheel cylinder) to allow controlled amounts of brake fluid to escape the system. While "bleeding" may appear to be a somewhat graphic term, it aptly describes the release of a vital fluid.
The same bleeding procedure applies to a hydraulic clutch.
It is possible for car owners to perform various procedures in servicing their own cars. This section will outline one possible procedure for brake-bleeding that can be utilized when servicing a car. Note that unless the master cylinder is being replaced, this procedure is the same regardless of whether the vehicle is equipped with ABS or not.
The following tools are necessary:
- Box-end wrench suitable for your car’s bleeder screws. An offset head design usually works best.
- Extra brake fluid (about 1 pint if you are just bleeding, about 3 if you are completely replacing).
- 12-inch long section of clear plastic tubing, ID sized to fit snugly over your car’s bleeder screws.
- Disposable bottle for waste fluid.
- One can of brake cleaner.
- One assistant (to pump the brake pedal).
Vehicle Preparation and Support
- Loosen the lug nuts of the road wheels and place the entire vehicle on jackstands. Be sure that the car is firmly supported before going ANY further with this procedure!
- Remove all road wheels.
- Install one lug nut backward at each corner and tighten the nut against the rotor surface. Note that this step is to limit caliper flex that may distort pedal feel.
- Open the hood and check the level of the brake fluid reservoir. Add fluid as necessary to ensure that the level is at the MAX marking of the reservoir. Do not let the reservoir become empty at any time during the bleeding process.
- Begin at the corner furthest from the driver and proceed in order toward the driver. (Right rear, left rear, right front, left front.) While the actual sequence is not critical to the bleed performance it is easy to remember the sequence as the farthest to the closest. This will also allow the system to be bled in such a way as to minimize the amount of potential cross-contamination between the new and old fluid.
- Locate the bleeder screw at the rear of the caliper body (or drum brake wheel cylinder.) Remove the rubber cap from the bleeder screw – and don’t lose it!
- Place the box-end wrench over the bleeder screw. An offset wrench works best – since it allows the most room for movement.
- Place one end of the clear plastic hose over the nipple of the bleeder screw.
- Place the other end of the hose into the disposable bottle.
- Place the bottle for waste fluid on top of the caliper body or drum assembly. Hold the bottle with one hand and grasp the wrench with the other hand.
- Instruct the assistant to "apply." The assistant should pump the brake pedal three times, hold the pedal down firmly, and respond with "applied." Instruct the assistant not to release the brakes until told to do so.
- Loosen the bleeder screw with a brief ¼ turn to release fluid into the waste line. The screw only needs to be open for one second or less. (The brake pedal will "fall" to the floor as the bleeder screw is opened. Instruct the assistant in advance not to release the brakes until instructed to do so.)
- Close the bleeder screw by tightening it gently. Note that one does not need to pull on the wrench with ridiculous force. Usually just a quick tug will do.
- Instruct the assistant to "release" the brakes. Note: do NOT release the brake pedal while the bleeder screw is open, as this will suck air back into the system!
- The assistant should respond with "released."
- Inspect the fluid within the waste line for air bubbles.
- Continue the bleeding process (steps 11 through 16) until air bubbles are no longer present. Be sure to check the brake fluid level in the reservoir after bleeding each wheel! Add fluid as necessary to keep the level at the MAX marking. (Typically, one repeats this process 5-10 times per wheel when doing a ‘standard’ bleed.)
- Move systematically toward the driver – right rear, left rear, right front, left front - repeating the bleeding process at each corner. Be sure to keep a watchful eye on the brake fluid reservior and keep it full at all times.
- When all four corners have been bled, spray the bleeder screw (and any other parts that were moistened with spilled or dripped brake fluid) with brake cleaner and wipe dry with a clean rag. (Leaving the area clean and dry will make it easier to spot leaks through visual inspection later!) Try to avoid spraying the brake cleaner DIRECTLY on any parts made of rubber or plastic, as the cleaner can make these parts brittle after repeated exposure.
- Test the brake pedal for a firm feel. (Bleeding the brakes will not necessarily cure a "soft" or "mushy" pedal – since pad taper and compliance elsewhere within the system can contribute to a soft pedal. But the pedal should not be any worse than it was prior to the bleeding procedure!)
-  - One of many useful links on the web for amateur mechanics.
-  - A page written by professional racers on maintaining and improving brake performance