Chevrolet Nova/ChevyII 1962-1979

Pressure Differential Warning Switch and Combination Valve


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Fig. Fig. 1 View of a common combination valve

This valve, mounted below the master cylinder, has both brake lines from the master cylinder connected to it and provides three functions on disc brake systems:

It meters or delays disc brake application until the shoes of the rear drum brakes contacts the drum.

It activates the brake failure warning light on the dash in the event of low pressure in one of the separate brake hydraulic systems.

Finally, it acts as a proportioning valve which reduces rear brake hydraulic pressure during panic stop situations. This delays rear wheel skidding due to weight transfer to the front wheels.

Models with front drum brakes have a similar unit, however it provides only for activation of the brake failure warning light.

Both the pressure differential warning switch and the combination valve are nonadjustable and nonserviceable. They must be replaced if defective.


  1. Disconnect the battery negative cable.
  3. Disconnect the electrical lead from the switch.
  5. Place rags under the unit to absorb any spilled brake fluid.
  7. Clean any dirt from the hydraulic lines and the switch/valve assembly. Disconnect the hydraulic lines from the assembly. If necessary, loosen the line connections at the master cylinder. Tape the open line ends to prevent the entrance of dirt.
  9. Remove the mounting screws and remove the switch/valve assembly.

To install:

  1. Make sure that the new unit is clean and free of dust and lint. If in doubt, wash the new unit in clean brake fluid.
  3. Place the new unit in position and install it to its mounting bracket with screws.
  5. Remove the tape from the hydraulic lines and connect them to the unit. If necessary, tighten the line connections at the master cylinder.
  7. Connect the electrical lead.
  9. Connect the battery negative cable.
  11. Bleed the brake systems.