4WAL Bleeding Procedure

*** THIS PROCEDURE HAS BEEN UPDATED BY TSB. #91-5E-23 (GMC) AND #91-74-5 (CHEVROLET), SEPTEMBER, 1990



Note:

Do not use D.O.T. 5 (silicone) brake fluid. System damage may result. It is not necessary to install the bleed valve tools on the EHCU or open the side internal bleed screws, if you are not replacing the EHCU. The EHCU does not have to be bled, unless it is replaced.




Caution:

Failure to follow the system bleeding procedure accurately will result in immediate soft pedal or soft pedal after the first antilock stop, as well as stored codes. The ignition switch must be off throughout bleeding to prevent codes from being set.




It is necessary to bleed the brake system whenever air is introduced into the system. If the EHCU (Electro-Hydraulic Control Unit) module has not been replaced normal brake system bleeding procedures may be used. If the EHCU has been replaced, or if it is suspected that air has been introduced into the EHCU the following steps should be taken.

Important

THIS REVISED PROCEDURE ELIMINATES THE NEED TO MANUALLY BLEED THE EHCU. THE FUNCTION TEST WILL PURGE THE MODULE OF ANY TRAPPED AIR.

1. Bleed the brake system in the conventional manner using either vacuum, pressure or manual bleeding, (Refer to VACUUM, PRESSURE OR MANUAL BLEEDING PROCEDURES). Technicians should always maintain a full master cylinder both during and after bleeding.


2. Perform THREE (3) system funtion tests, (Refer to ECU FUNCTIONAL TEST under DIAGNOSIS AND TESTING PROCEDURES).


Important

DURING THE FUNCTION TEST THE BRAKE PEDAL MUST BE FIRMLY DEPRESSED. THIS WILL PURGE ANY AIR
TRAPPED IN THE EHCU HIGH PRESSURE ACCUMULATOR, LOW PRESSURE ACCUMULATOR AND PUMP CIRCUITS.

3. Re-Bleed the vehicle brake system as instructed in step one to purge any air introduced to the brake lines from the EHCU purging.

4. Confirm that no soft or spongy pedal action is observed. Repeat steps 1 through 4 if pedal response indicates air remains in the system.

5. If "Soft Pedal" persists, proceed with 4WAL ALTERNATIVE BLEEDING PROCEDURE.