GM Full-Size Trucks 1988-1998 Repair Information

Combination Valve

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This valve is used on all models with disc brakes. The valve itself is a combination of:

  1. The metering valve, which will not allow the front disc brakes to engage until the rear brakes contact the drum.
  2.  
  3. The failure warning switch, which notifies the driver if one of the systems has a leak.
  4.  
  5. The proportioner which limits rear brake pressure and delays rear wheel skid.
  6.  

REMOVAL & INSTALLATION



R/V Series

See Figure 1

  1. Disconnect the hydraulic lines and plug them to prevent dirt from entering the system.
  2.  



Click image to see an enlarged view

Fig. Fig. 1: Combination valve mounting-R/V Series

  1. Disconnect the warning switch harness.
  2.  
  3. Unfasten the retaining bolts and remove the valve.
  4.  
  5. Install the valve and tighten the retainers to 150 inch lbs. (17 Nm).
  6.  
  7. Bleed the brake system.
  8.  

C/K Series

MANUAL BRAKES

See Figure 2

  1. Disconnect the brake lines at the valve. Plug or cap the lines and ports.
  2.  
  3. Unplug the switch wiring connector.
  4.  
  5. Remove the valve-to-bracket bolts.
  6.  

To install:
  1. Install the valve. Tighten the bolts to 20 ft. lbs. (27 Nm).
  2.  
  3. Bleed the system.
  4.  



Click image to see an enlarged view

Fig. Fig. 2: Combination valve-C/K Series

POWER BRAKES
  1. Disconnect the brake lines at the switch. Plug or cap the lines and ports.
  2.  
  3. Unplug the warning switch wiring connector.
  4.  
  5. Remove the anti-lock brake system control module from the bracket. See the procedure later in this section.
  6.  
  7. Remove the bolts attaching the ABS isolation/dump valve to the bracket. See the procedure later in this section.
  8.  
  9. Remove the nuts that attach the master cylinder and bracket to the booster.
  10.  
  11. Remove the bracket and combination valve assembly.
  12.  

To install:
  1. Position the bracket/valve assembly and install the master cylinder-to-booster nuts. Tighten the nuts to 20 ft. lbs. (27 Nm).
  2.  
  3. Install the ABS isolation/dump valve nuts. Tighten them to 21 ft. lbs. (24 Nm).
  4.  
  5. Install the ABS control module.
  6.  
  7. Connect the wiring.
  8.  
  9. Connect the hydraulic lines.
  10.  
  11. Bleed the system.
  12.  

SWITCH CENTERING



Whenever work on the brake system is done it is possible that the brake warning light will come on and refuse to go off when the work is finished. In this event, the switch must be centered.

  1. Raise and support the truck.
  2.  
  3. Attach a bleeder hose to the rear brake bleed screw and immerse the other end of the hose in a jar of clean brake fluid.
  4.  
  5. Be sure that the master cylinder is full.
  6.  
  7. When bleeding the brakes, the pin in the end of the metering portion of the combination valve must be held in the open position (with the tool described in the brake bleeding section installed under the pin mounting bolt). Be sure to tighten the bolt after removing the tool.
  8.  
  9. Turn the ignition key ON. Open the bleed screw while an assistant applies heavy pressure on the brake pedal. The warning lamp should light. Close the bleed screw before the helper releases the pedal.
  10.  
  11. To reset the switch, apply heavy pressure to the pedal. This will apply hydraulic pressure to the switch which will re-center it.
  12.  
  13. Repeat Step 5 for the front bleed screw.
  14.  
  15. Turn the ignition OFF and lower the truck.
  16.  

If the warning lamp does not light during Step 5, the switch is defective and must be replaced.

 
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