GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Bleeding the 4WAL Brake System


See Figures 1 and 2

The EHCU module is the one component which adds to the complexity of bleeding the 4WAL brake systems. For the most part the system is bled in the same manner as the non-ABS vehicles. Refer to the procedure earlier in this section for details. But because of the EHCU's complex internal valving additional steps are necessary if the unit has been replaced or if it is suspected to contain air. These bleeding steps are NOT necessary if the only connection/fitting(s) opened were downstream of the unit. These steps may or may not be necessary after master cylinder replacement. If in doubt (or without the necessary special tools) thoroughly bleed the system and see if a firm brake pedal can be obtained, if not, the EHCU must be bled as well.

As with the RWAL brake system, the use of a power bleeder is recommended, but the system may also be bled manually. If a power bleeder is used, it must be of the diaphragm type and provide isolation of the fluid from air and moisture.

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Fig. Fig. 1: The EHCU valve and the internal bleeder location

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Fig. Fig. 2: Depressing the combination valve

Do not pump the pedal rapidly when bleeding; this can make the circuits very difficult to bleed. Instead, press the brake pedal slowly 1 time and hold it down while bleeding takes place. Tighten the bleeder screw, release the pedal and wait 15 seconds before repeating the sequence. Because of the length of the brake lines and other factors, it may take 10 or more repetitions of the sequence to bleed each line properly. When necessary to bleed all 4 wheels, the correct order is right rear, left rear, right front and left front.

Do not move the vehicle until a firm brake pedal is achieved. Failure to properly bleed the system may cause impaired braking and the possibility of injury and/or property damage

If the EHCU requires bleeding, the following procedure may be used to free all trapped air from the component. Three combination valve depressor tools and a scan tool are required. The combination valve depressor tools are used to hold the internal passages (combination valve and EHCU bleed accumulator bleed stems open allowing the entire system to be completely bled.

The combination valve tools are relatively inexpensive and should be available from various aftermarket companies. Although a homemade tool may suffice, DO NOT attempt to fabricate a homemade tool unless you are CERTAIN it will not damage the valve/bleed stem by over-extension.

Finally, remember to always bleed the 4WAL brake system with the ignition OFF to prevent setting false trouble codes.

1991-92 MFI-TURBO

The EHCU used on the 1991-92 MFI-Turbo is equipped with the internal bleeders AND a pair of external bleeder screws. These external bleeders look like normal brake bleeders and are found on top of the unit. Like any bleeder screw, they MUST remain closed when the unit is not pressurized.

The Internal Bleed Valves on either side of the unit must be opened 1 / 4 - 1 / 2 turn before bleeding begins. These valves open internal passages within the unit. The valve located on the left side (nearest the fender) is used for the rear brake section, while the valve on the right (nearest the engine) is used for the front brakes. Actual bleeding is performed at the two bleeders on the top of the EHCU module. The bleeders must not be opened when the system is not pressurized. The ignition switch must be OFF or false trouble codes may be set.

  1. Make sure the ignition is in the OFF position to prevent setting false trouble codes.
  3. Open the internal bleed valves 1 / 4 - 1 / 2 turn each.
  5. Install J-35856 or equivalent combination valve depressor tool on the left accumulator bleed stem of the EHCU. Install one tool on the right bleed stem and install the third tool on the combination valve (rear).
  7. Inspect the fluid level in the master cylinder, filling if needed.
  9. Have an assistant slowly depress the brake pedal and hold it down.
  11. Open the left bleeder on top of the unit. Allow fluid to flow until no air is seen or until the brake pedal bottoms.
  13. Close the left bleeder, then have your assistant release the pedal slowly and wait 15 seconds.
  15. Repeat these steps starting with depressing the brake pedal (including the 15 second pause), until no air is seen in the fluid.
  17. Tighten the left internal bleed valve to 60 inch lbs. (7 Nm).
  19. Bleed air from the right bleeder screw on top of the EHCU in the same manner as the left screw.
  21. When bleeding of the right port is complete, tighten the right internal bleed valve to 60 inch lbs. (7 Nm).
  23. Remove the 3 special combination valve tools.
  25. Check the master cylinder fluid level, refilling as necessary.
  27. Bleed the individual brake circuits at each wheel. Again, please refer to the hydraulic brake bleeding procedure located earlier in this section.
  29. Switch the ignition ON and use the hand scanner tool to perform 3 function tests on the system.
  31. Evaluate the brake pedal feel and repeat the bleeding procedure if it is not firm.
  33. Carefully test drive the vehicle at moderate speeds; check for proper pedal feel and brake operation. If any problem is noted in feel or function, repeat the entire bleeding procedure.