GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

Bleeding the Brake System

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1994-96 vcm-ABS systems and 1994-95 4wal abs systems

See Figures 1, 2 and 3

The EHCU/BPMV 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 non-ABS system brake bleeding procedure earlier in this section for details. But because of the EHCU/BPMV'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/BPMV 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/BPMV 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.


CAUTION
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/BPMV requires bleeding, the following procedures may be used to free all trapped air from the component. The combination valve depressor tools are used to hold the internal passages (combination valve and EHCU/BPMV 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.

  1. Make sure the ignition is in the OFF position to prevent setting false trouble codes.
  2.  
  3. If necessary, properly bleed the master cylinder assembly as directed in the hydraulic brake bleeding procedure found earlier in this section. Check and add additional fluid, as necessary.
  4.  
  5. Open the internal bleed valves 1 / 4 - 1 / 2 turn each.
  6.  
  7. Install one J-39177 or equivalent combination valve depressor tool on the left accumulator bleed stem of the EHCU. Install one tool on the right accumulator bleed stem and install the third tool on the combination valve.
  8.  
  9. Properly bleed the wheel cylinders and calipers. For details, please refer to the hydraulic brake bleeding procedure located earlier in this section.
  10.  
  11. Remove the 3 special tools.
  12.  
  13. Check the master cylinder fluid level, refilling as necessary.
  14.  
  15. Switch the ignition ON (engine not running) and use a hand scanner to perform 6 function tests on the system.
  16.  
  17. Repeat the wheel cylinder and caliper bleeding procedure to remove all air that was purged from the BPMV during the function tests.
  18.  
  19. Check for a firm brake pedal. If necessary, repeat the entire procedure until a firm pedal is obtained.
  20.  
  21. 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.
  22.  

1996-99 4wal ABS system

The manual bleeding procedure is preferred on these models. Pressure bleeding may be used may be used for base brake bleeding only and is not suitable on these ABS systems.

To bleed the system, a length of clear neoprene bleeder hose, bleeder wrenches and a clear bleeder bottle (old glass jar or drink bottle will suffice). A scan tool should also be used to run the system through functional tests.

  1. Clean the top of the master cylinder, remove the cover and fill the reservoirs with clean fluid. To prevent squirting fluid, and possibly damaging painted surfaces, install the cover during the procedure, but be sure to frequently check and top off the reservoirs with fresh fluid.
  2.  


WARNING
Never reuse brake fluid, which has been bled from the system.


WARNING
Do not allow brake fluid to spill on or come in contact with the vehicle's finish as it will remove the paint. In case of a spill, immediately flush the area with water.

  1. The master cylinder must be bled first if it is suspected to contain air. If the master cylinder was removed and bench bled before installation it must still be bled, but it should take less time and effort. Bleed the master cylinder as follows:
    1. Position a container under the master cylinder to catch the brake fluid.
    2.  
    3. Loosen the front brake line at the master cylinder and allow the fluid to flow from the front port.
    4.  
    5. Have a friend depress the brake pedal slowly and hold (air and/or fluid should be expelled from the loose fitting). Tighten the line, then release the brake pedal and wait 15 seconds. Loosen the fitting and repeat until all air is removed from the master cylinder bore.
    6.  
    7. When finished, tighten the line fitting.
    8.  
    9. Repeat the sequence at the master cylinder rear pipe fitting.
    10.  

  2.  

During the bleeding procedure, make sure your assistant does NOT release the brake pedal while a fitting is loosened or while a bleeder screw is opening. Air will be drawn back into the system.

  1. Check and refill the master cylinder reservoir.
  2.  

Remember, if the reservoir is allowed to empty of fluid during the procedure, air will be drawn into the system and bleeding procedure must be restarted at the master cylinder assembly.

  1. All calipers and wheel cylinders must be bled in the proper sequence:
    1. Right rear
    2.  
    3. Left rear
    4.  
    5. Right front
    6.  
    7. Left front
    8.  

  2.  
  3. Check the master cylinder after every four-to-six strokes of the brake pedal and replenish as necessary. This will avoid running the system dry.
  4.  
  5. Bleed the individual calipers or wheel cylinders as follows:
    1. Place a suitable wrench over the bleeder screw and attach a clear plastic hose over the screw end. Be sure the hose is seated snugly on the screw or you may be squirted with brake fluid.
    2.  

  6.  

Be very careful when bleeding wheel cylinders and brake calipers. The bleeder screws often rust in position and may easily break off if forced. Installing a new bleeder screw will often require removal of the component and may include overhaul or replacement of the wheel cylinder/caliper. To help prevent the possibility of breaking a bleeder screw, spray it with some penetrating oil before attempting to loosen it.

    1. Submerge the other end of the tube in a transparent container of clean brake fluid.
    2.  
    3. Loosen the bleed screw, then have a friend apply the brake pedal slowly through its full travel and hold. Tighten the bleed screw, release the brake pedal and wait 15 seconds. Repeat the sequence (including the 15 second pause) until all air is expelled from the caliper or cylinder.
    4.  
    5. Tighten the bleed screw when finished.
    6.  


  1. Repeat the procedure at each wheel until approximately one pint of fluid has been bled from each wheel. Clean fluid should be present at each wheel bleed screws.
  2.  

On 1997 and later models, do not run the function test after combination valve or tube adapter replacement except on S/T trucks.

  1. If any component is replaced, which may have caused air to enter the BPMV, use a suitable scan tool to run the function tests four times while applying the brake pedal firmly.
  2.  

Set the parking brake when running the function tests.

  1. Re-bleed all four wheels using the procedures described above after running the function tests so that all air is expelled from the brake system.
  2.  
  3. Check the pedal feel before attempting to move the vehicle and re-bleed the system as often as necessary to obtain the proper pedal feel.
  4.  

 
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