GM Full Size Vans 1967-1986 Repair Guide

Combination Valve



See Figure 1

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Fig. Fig. 1: Common combination valve location and mounting

This valve is used on all models with disc brakes. It is non-adjustable and non-serviceable. It can be found by following the lines from the master cylinder. The combination valve itself contains a metering valve that restricts flow to the front brakes until the rear brakes overcome the force of their retracting springs to prevent front brake lockup, a pressure differential warning switch which activates a warning light if either the front or rear hydraulic circuit is losing pressure, and a proportioning valve which limits hydraulic pressure to the rear brakes to prevent rear wheel lockup.

The pressure differential warning switch will reset itself automatically when the brakes are used after a malfunction causing the warning light to go on has been corrected.

When the brake hydraulic system is bled of air, the metering valve pin on the end of the combination valve must be hold in a slight amount to allow fluid flow to the front brakes.


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.
  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.
  5. Be sure that the master cylinder is full.
  7. When bleeding the brakes, the pin in the end of the metering portion of the combination valve must be hold 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.
  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.

To reset the switch, apply heavy pressure to the pedal. This will apply hydraulic pressure to the switch which will re-center it.

  1. Repeat Step 5 for the front bleed screw.
  3. Turn the ignition OFF and lower the truck.

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