Brake Proportioning/Combination Valve: Testing and Inspection

On vehicles with Bosch Anti-lock Brake System, ABS-3, Bendix Anti-Lock 6 or Bendix Anti-Lock 10, refer to ANTILOCK BRAKE SYSTEM.

If premature rear wheel skid occurs on hard brake application, it could be an indication of a malfunctioning proportioning valve unit.

The proportioning valve is designed with two separate systems. One half controls the right rear brake, and the other half controls the left rear brake. A road test to determine which rear brake slides first must be performed.

Fig. 26 Type 2, Combination Valve Pressure Gauge Test Connections. Right Wheel Malfunction:

1. To test proportioning valve when right rear wheel slides first, leave front brakes connected to valve, then proceed as follows:
a. Install gauge set No. C-4007A, as shown. A special adapter may be fabricated from a short piece of brake tube and two 3/8 X 24 tube nuts.
b. Bleed the hose and gauge set.
c. While watching pressure gauges, have an assistant depress the brake pedal. Note gauge readings, then refer to Brakes and Traction Control/Brakes/Specifications/Mechanical.

Fig. 27 Type 2, Combination Valve Pressure Gauge Test Connections. Left Wheel Malfunction:

2. To test proportioning valve when left rear wheel slides first, leave front brakes connected to valve, then proceed as follows:
a. Install gauge set No. C-4007A, as shown. A special adapter may be fabricated from a 7/16 X 24 tube nut, a short piece of brake tube and a 3/8 X 24 tube nut.
b. Bleed hose and gauge set.
c. While watching pressure gauges, have an assistant depress the brake pedal. Note gauge readings, then refer to SPECIFICATIONS/MECHANICAL SPECIFICATIONS/HYDRAULIC BRAKE CONTROLS SPECIFICATIONS.
3. If pressure readings are not as specified for left or right rear wheel, replace the combination valve noting the following:
a. After installing a new combination valve, bleed hydraulic brake system, refer to Brakes and Traction Control/Brakes/Service and Repair.
b. Upon completion of bleeding hydraulic brake system, reset brake warning switch by depressing brake pedal with moderate force. Brake warning lamp will turn off when switch is reset.