GM Full-Size Trucks 1970-1979 Repair Guide

Combination Valve


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

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


  1. Disconnect the hydraulic lines and plug to prevent dirt from entering the system.
  3. Disconnect the warning switch harness.
  5. Remove the retaining bolts and remove the valve.
  7. Install in reverse of removal and bleed the brake system.


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 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.
  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 recenter 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.