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 after it has been properly connected to be sure that all air is expelled from the brake cylinders and lines.
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. keep the master cylinder reservoir filled with brake fluid during bleeding operation. Never use brake fluid that has been drained from the hydraulic system, no matter how clean it is.
It will not be necessary to centralize the pressure differential valve after a brake system failure has been corrected and the hydraulic system has been bled. The valve is self-centering.
There three methods for bleeding the air out of the brake system:
- Gravity bleeding. Remove the master cylinder cover and gasket, then fill the reservoir with approved brake fluid. Open the bleeder screws and allow fluid and air to drain until the stream of fluid is free of air bubbles. Use a liberal amount of oil absorbent material, shop towels or rags to soak up the brake fluid as it drains. Dispose of the mess properly. It does not make good garden mulch.
- Pedal bleeding. This is the most widely-known service procedure requiring two people to perform. Have an assistant pump and hold the pedal to build up brake pressure, then open the bleeder valve until fluid squirts out. Remember not to stand directly in front of the valve when cracking it open. Be careful not to pump the master cylinder dry.
- Pressure bleeding. This requires the use of pressure bleeding equipment not generally available to the general public at a price that makes it cost-effective to own. Leave this method to your professional mechanic. If you already have pressure bleeding equipment, we assume you know how to use it. If so, don't forget to lock the brake valve block stem open while pressure bleeding the system.
STANDARD BRAKE BLEEDING PROCEDURE
See Figures 1, 2, 3 and 4
- 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 of the edge of the reservoir.
- Clean off the bleeder screws at the wheel cylinders and calipers.
- Attach the length of rubber hose over the nozzle of the bleeder screw at the wheel to be done first. Place the other end of the hose in a glass jar, submerged in brake fluid.
- Open the bleed screw valve 1 / 2 - 3 / 4 turn.
- Have an assistant slowly depress the brake pedal. Close the bleeder screw valve and tell your assistant to allow the brake pedal to return slowly. Continue this pumping action to force any air out of the system. When bubbles cease to appear at the end of the bleeder hose, close the bleed valve and remove the hose.
- Check the master cylinder fluid level and add fluid accordingly. Do this after bleeding each wheel.
- Repeat the bleeding operation at the remaining 3 wheels, ending with the one closest to the master cylinder. Fill the master cylinder reservoir.
MASTER CYLINDER BLEEDING
- Fill the master cylinder reservoirs.
- Place absorbent rags under the fluid lines at the master cylinder.
- Have an assistant depress and hold the brake pedal.
- With the pedal held down, slowly crack open the hydraulic line fitting, allowing the air to escape. Close the fitting and have the pedal released.
- Repeat Steps 3 and 4 for each fitting until all the air is released.