See Figures 1 and 2
The hydraulic brake system must be free of air to operate properly. Air can enter the system when hydraulic parts are disconnected for servicing or replacement, or when the fluid level in the master cylinder reservoirs is very low. Air in the system will give the brake pedal a spongy feeling upon application.
The quickest and easiest of the two ways for system bleeding is the pressure method, but special equipment is needed to externally pressurize the hydraulic system. The other, more commonly used method of brake bleeding is done manually.
- Master cylinder. If the cylinder is not equipped with bleeder screws, open the brake line(s) to the wheels slightly while pressure is applied to the brake pedal. Be sure to tighten the line before the brake pedal is released. The procedure for bench bleeding the master cylinder is covered below.
- Pressure Differential Valve: If equipped with a bleeder screw.
- Front/Back Split Systems: Start with the wheel farthest away from the master cylinder, usually the right rear wheel. Bleed the other rear wheel then the right front and left front.
If the vehicle is equipped with anti-lock brakes, the electro-hydraulic valve must also be bled. It is not necessary to energize the valve to bleed it.
- Clean the bleed screw at each wheel.
- Start with the wheel farthest from the master cylinder (right rear), or the left rear if the right side wheel cylinder is not equipped with a bleeder..
- Attach a small rubber hose to the bleed screw and place the end in a container of clear brake fluid.
- Fill the master cylinder with brake fluid. (Check often during bleeding). Have an assistant slowly pump up the brake pedal and hold pressure.
- Open the bleed screw about one-quarter turn, press the brake pedal to the floor, close the bleed screw and slowly release the pedal. Continue until no more air bubbles are forced from the cylinder on application of the brake pedal.
- Repeat procedure on remaining wheel cylinders and calipers, still working from cylinder/caliper farthest from the master cylinder.
Master cylinders equipped with bleed screws may be bled independently. When bleeding the Bendix-type dual master cylinder it is necessary to solidly cap one reservoir section while bleeding the other to prevent pressure loss through the cap vent hole.
FLUSHING HYDRAULIC BRAKE SYSTEMS
Hydraulic brake systems must be totally flushed if the fluid becomes contaminated with water, dirt or other corrosive chemicals. To flush, simply bleed the entire system until all fluid has been replaced with the correct type of new fluid.