Ford Thunderbird/Cougar 1983-1997 Repair Guide

Pressure Differential Warning Valve


A pressure differential warning valve is incorporated in the hydraulic system. The signal from this valve consists of a warning light on the dashboard activated by a differential pressure switch located below the master cylinder. The signal indicates a hydraulic pressure differential between the front and rear brakes of 80-150 psi, and should warn the driver that a hydraulic failure has occurred.

After repairing and bleeding any part of the hydraulic system the warning light may remain on due to the pressure differential valve remaining in the off-center position. To centralize the valve a pressure difference must be created in the opposite branch of the hydraulic system that was repaired or bled last.

Front wheel balancing of cars equipped with disc brakes may also cause a pressure differential in the front branch of the system.


  1. Turn the ignition to either the ACC or ON position.
  3. Check the fluid level in the master cylinder reservoirs. Fill to within 1 / 4 in. (6mm) of the top if necessary.
  5. Depress the brake pedal firmly. The valve will centralize itself causing the brake warning light to go out.
  7. Turn the ignition off.
  9. Prior to driving the vehicle, check the operation of the brakes and obtain a firm pedal.