The purpose of bleeding the brakes is to expel air trapped in the hydraulic system. The system must be bled whenever the pedal feels spongy, indicating that air, which is compressible, has entered the system. It must also be bled whenever the system has been opened or repaired. You will need a helper for this job.
Never reuse brake fluid which has been bled from the system.
The sequence for bleeding is right rear, left rear, right front, left front. The usual procedure is to bleed at the points farthest from the master cylinder first.
- Clean all dirt from around the master cylinder reservoir caps. Remove the caps and fill the master cylinder to the proper level with clean, fresh brake fluid meeting DOT 3 specifications.
Brake fluid picks up moisture from the air, which reduces its effectiveness and causes brake line corrosion. Don't leave the master cylinder or the fluid container open any longer than necessary. Be careful not to spill brake fluid on painted surfaces. Wipe up any spilled fluid immediately and rinse the area with clear water.
- Clean all the bleeder screws. You may want to give each one a shot of penetrating solvent to loosen it up. Seizure is a common problem with bleeder screws, which then break off, sometimes requiring replacement of the part to which they are attached (e.g., calipers, etc.).
- Attach a length of clear vinyl tubing to the bleeder screw on the wheel cylinder. Insert the other end of the tube into a clear, clean plastic jar half filled with brake fluid.
- Have your helper slowly depress the brake pedal. As this is done, open the bleeder screw 1 / 3 - 1 / 2 of a turn, and allow the fluid to run through the tube. Close the bleeder screw before the pedal reaches the end of its travel. Have your assistant slowly release the pedal. Repeat this process until no air bubbles appear in the expelled fluid.
Some front drum brakes have two hydraulic cylinders and two bleeder screws. Both cylinders must be bled.
- Repeat the procedure on the other three brakes, checking the fluid level in the master cylinder reservoirs often. Do not allow the reservoirs to run dry, or the bleeding process will have to be repeated.