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.
See Figures 1, 2 and 3
- 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.
See Figure 4
- Clean the bleed screw at each wheel.
- Start with the wheel farthest from the master cylinder (right rear).
- 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.
PRESSURE BLEEDING DISC BRAKES
Pressure bleeding equipment should be diaphragm type+acing a diaphragm between the pressurized air supply and the brake fluid. This prevents moisture and other contaminants from entering the hydraulic system.
Some front disc/rear drum equipped vehicles use a metering valve which closes off pressure to the front brakes under certain conditions. These systems contain manual release actuators, which must be engaged to pressure bleed the front brakes.
- Connect the tank hydraulic hose and adapter to the master cylinder.
- Close hydraulic valve on the bleeder equipment.
- Apply air pressure to the bleeder equipment.
- Open the valve to bleed air out of the pressure hose to the master cylinder.
Never bleed this system using the secondary piston stopscrew on the bottom of many master cylinders.
- Open the hydraulic valve and bleed each wheel cylinder and caliper. Bleed rear brake system first when bleeding both front and rear systems.
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.
BENCH BLEEDING MASTER CYLINDER
See Figure 5
Bench bleeding the master cylinder before installing it on the truck reduces the possibility of getting air into the lines.
- Connect two short pieces of brake line to the outlet fittings, bend them until the free end is below the fluid level in the master cylinder reservoirs.
- Fill the reservoirs with fresh brake fluid. Pump the piston until no more air bubbles appear in the reservoir(s).
- Disconnect the two short lines, refill the master cylinder and securely install the cylinder cap(s).
- Install the master cylinder on the truck. Attach the lines but do not completely tighten them. Force any air that might have been trapped in the connection by slowly depressing the brake pedal. Tighten the lines before releasing the brake pedal.