Brake Proportioning/Combination Valve: Description and Operation

Proportioning Valves For Non ABS Equipped Vehicles:

This vehicle uses proportioning valves that are inline with the rear brake tubes. The in-line proportioning valves used on this vehicle, replace the combination valve used in prior designs. With this new design, the chassis brake tubes connect directly from the master cylinder or Hydraulic Control Unit (HCU) to the brake flex hose. Vehicles not equipped with ABS use a master cylinder incorporating the standard type compensating port design. Vehicles which are equipped with ABS, use a master cylinder having a dual center valve design. In addition, the ABS master cylinder is a 2 outlet port design and the non-ABS master cylinder is a four outlet design. The non ABS brake system uses two proportioning valves in-line with each of the rear wheel chassis brake tubes. The non ABS equipped proportioning valves are located on the front suspension crossmember in the same area as the hydraulic unit on ABS equipped vehicles.

Proportioning Valve Location For Antilock Brake Equipped Vehicles:

On vehicles equipped with antilock brakes, the proportioning valves are located under the vehicle on the passenger side just rearward of the passenger seat. The proportioning valves mount directly in the chassis brake tube going to each of the vehicles rear brakes.

Proportioning valves balance front to rear braking by controlling at a given ratio, the increase in rear brake system hydraulic pressure above a preset level (split point). Under light pedal application, the proportioning valve allows full hydraulic pressure to be applied to the rear brakes.

Proportioning Valve With ABS And Non-ABS TLEV:

There are two proportioning valve assemblies used in each vehicle. During any service procedures identify valve assemblies by supplier part number and or the bar code label and stamp identification band. This vehicle's brake system uses different calibrations for the proportioning valves. For a vehicle equipped with front disc and rear drum brakes the split point is 600 psi and the slope is 0.59. For a vehicle equipped with front and rear disc brakes the split point is 500 psi and the slope is 0.43.