Brake Proportioning/Combination Valve: Description and Operation

These vehicles have a pressure differential control valve that actuates the brake warning light. When a pressure differential occurs within the valve, pressure from high pressure side forces pressure toward low pressure side, depressing lamp switch plunger which illuminates brake warning light.
Some 1985---87 models use a height sensing brake proportioning valve to regulate rear brake hydraulic pressure when the vehicle is in the empty loaded condition and provide full braking pressure to the rear brakes when vehicle is carrying a full load.
This valve is located on the No. 5 crossmember and is activated through a linkage system connected to the rear axle housing cover. Pressure on the lever from the vehicle load will activate the height sensing valve to regulate hydraulic pressure to apply rear brakes.