The hydraulic brake system must be bled any time one of the lines is disconnected or any time air enters the system. If the brake pedal feels spongy upon application, and goes almost to the floor but regains height when pumped, air has entered the system. It must be bled out. Check for leaks that would have allowed the entry of air and repair them before bleeding the system. The correct bleeding sequence is; right rear, left rear, right front and left front.
See Figures 1, 2 and 3
This method of bleeding requires 2 people, one to depress the brake pedal and the other to open the bleeder screws.
The following procedure requires the use of a clear vinyl hose, a glass jar and clean brake fluid.
- Clean the top of the master cylinder, remove the cover and fill the reservoirs with clean fluid. To prevent squirting fluid, replace the cover.
On vehicles equipped with front disc brakes, it will be necessary to hold in the metering valve pin during the bleeding procedure. The metering valve is located beneath the master cylinder and the pin is situated under the rubber boot on the end of the valve housing. This may be tapped in or held by an assistant.
- Fill the master cylinder with brake fluid.
- Install a box end wrench onto the bleeder screw on the right rear wheel.
- Attach a length of small diameter, clear vinyl tubing to the bleeder screw. Submerge the other end of the tubing in a glass jar partially filled with clean brake fluid. Make sure the tube fits on the bleeder screw snugly or you may be squirted with brake fluid when the bleeder screw is opened.
- Have your assistant slowly depress the brake pedal. As this is done, open the bleeder screw 1 / 2 turn and allow the fluid to run through the tube. Close the bleeder screw, then return the brake pedal to its fully released position.
- Repeat this procedure until no bubbles appear in the jar. Refill the master cylinder.
- Repeat this procedure on the left rear, right front and the left front wheels, in that order. Periodically, refill the master cylinder so that it does not run dry.
- If the brake warning light is ON, depress the brake pedal firmly. If there is no air in the system, the light will go OFF.
See Figures 4 and 5
The following procedure requires the use of the GM Brake Bleeder Adapter tool No. J-29567 or equivalent, and the GM Combination Valve Depressor tool No. J-35856 or equivalent.
- Using the GM Brake Bleeder Adapter tool No. J-29567 or equivalent, fill the pressure tank to at least 1 / 3 full of brake fluid. Using compressed air, charge the pressure tank to 20-25 psi. (138-172 kPa), then install it onto the master cylinder.
- Using the GM Combination Valve Depressor tool No. J-35856 or equivalent, install it onto the combination valve to hold the valve open during the bleeding operation.
- Bleed each wheel cylinder or caliper in the following sequence: right rear, left rear, right front and left front.
- Connect a hose from the bleeder tank to the adapter at the master cylinder, then open the tank valve.
- Attach a clear vinyl hose to the brake bleeder screw, then immerse the opposite end into a container partially filled with clean brake fluid.
- Open the bleeder screw 3 / 4 turn and allow the fluid to flow until no air bubbles are seen in the fluid, then close the bleeder screw.
- Repeat the bleeding process to each wheel.
- Inspect the brake pedal for sponginess and if necessary, repeat the entire bleeding procedure.
- Remove the depressor tool from the combination valve and the bleeder adapter from the master cylinder.
- Refill the master cylinder to the proper level with brake fluid.
See Figure 6
The rear wheel and 4-wheel anti-lock brakes are bled the same way as the standard brakes, pertaining to the master cylinder and each wheel cylinder. The difference is in the 4-wheel anti-lock system. The Electro-hydraulic Control Unit (EHCU) valve has to be bled after replacement only.
Use the 2 bleed screws on the EHCU valve for bleeding. There are also 2 bleeders on the front of the unit that look like normal brake bleeders. These are NOT the correct bleeders for bleeding the valve and they should not be turned.
- Bleed the calipers and wheel cylinder first.
- Install a valve depressor tool J-35856 onto the left high pressure accumulator bleed stem of the EHCU valve.
- Slowly depress the brake pedal one time and hold. Loosen the left bleeder screw 1 / 4 turn to purge the air from the EHCU valve.
- Tighten the bleeder screw to 60 inch lbs. (7 Nm) and slowly release the pedal.
- Wait 15 seconds, then repeat the sequence, purging the EHCU valve.
- Repeat steps 2-5 at the right side of the EHCU valve.
- Remove the valve depressor tool.