When any part of the hydraulic system has been disconnected for repair or replacement, air may get into the lines and cause spongy pedal action (because air can be compressed and brake fluid cannot). To correct this condition, it is necessary to bleed the hydraulic system so to be sure all air is purged.
Bleeding must start where the lines were disconnected. If lines were disconnected at the master cylinder, for example, bleeding must be done at that point before proceeding downstream.
When bleeding the brake system, bleed one brake cylinder at a time, beginning at the cylinder with the longest hydraulic line (farthest from the master cylinder) first. Failure to do so may result in more air being drawn into the lines.
If the existing system fluid seems dirty or if the vehicle has covered considerable mileage, it is recommended that the system be completely purged and refilled with fresh, clean fluid. The best way to start is to siphon the old fluid out of the master cylinder reservoir and fill it completely with fresh fluid.
Brake fluid tends to darken over time. This does not necessarily indicate contamination. Examine fluid closely for foreign matter.
The primary and secondary hydraulic brake systems are separate and are bled independently. During the bleeding operation, Do NOT allow the reservoir to run dry. Keep the master cylinder reservoir filled with brake fluid. Never use brake fluid that has been drained from the hydraulic system, no matter how clean it seems.
- Clean all dirt from around the master cylinder fill cap, remove the cap and fill the master cylinder with brake fluid until the level is within 1 / 4 in. (6mm) of the top edge of the reservoir.
- Clean the bleeder screws at all four wheels. The bleeder screws are located on the back of the brake backing plate (drum brakes) and on the top of the brake calipers (disc brakes).
- Bleeder screws should be protected with rubber caps. If they are missing, the oriface may easily become clogged with road dirt. If the screw refuses to bleed when loosened, remove it and blow clear. Aftermarket caps are readily available.
Manual bleeding requires two people and a degree of patience and cooperation.
- Follow the preparatory steps, above.
- Attach a length of rubber hose over the bleeder screw and place the other end of the hose in a glass jar, submerged in brake fluid.
- Have your assistant press down on the brake pedal, then open the bleeder screw 1 / 2 - 3 / 4 turn.
- The brake pedal will go to the floor.
- Close the bleeder screw-preferably before the pedal reaches the floor. Tell your assistant to allow the brake pedal to return slowly.
- Repeat these steps to purge all air from the system.
- When bubbles cease to appear at the end of the bleeder hose, close the bleeder screw and remove the hose. Check that the pedal is firm or at least firmer than it was when you started. If not, continue the procedure.
- Check the master cylinder fluid level and add fluid accordingly. Do this after bleeding each wheel.
- Repeat the bleeding operation at the remaining three wheels, ending with the one closet to the master cylinder.
- Fill the master cylinder reservoir to the proper level.
Vacuum bleeding can be carried out by one person. Since a good vacuum bleeder will normally move more fluid than a brake pedal stroke, this procedure is preferred. These tools are inexpensive and readily available at auto parts outlets.
- Follow the preparatory steps, above.
- Attach the vacuum bleeder according to the manufacturer's recommendations.
- Pump up the unit until maximum vacuum is reached. Loosen the bleeder screw slightly until bubbles and fluid issue forth. Close the screw before the vacuum is equalized.
- Repeat the procedure until fluid without bubbles issues from the bleeder screw.
- Keep a close check on master cylinder fluid level during this procedure as vacuum bleeders move considerable amounts of fluid.