Brake Proportioning/Combination Valve: Testing and Inspection

Fig. 4 Gauge hook-up for testing proportioning valve (typical):

The proportioning valve (when used), provides balanced braking action between front and rear brakes under a wide range of braking conditions. The valve regulates the hydraulic pressure applied to the rear wheel cylinders, thus limiting rear braking action when high pressures are required at the front brakes. In this manner, premature rear wheel skid is prevented.

TESTING PROPORTIONING VALVE
When a premature rear wheel slide is obtained on a brake application, it usually is an indication that the fluid pressure to the rear wheels is above the 50 % reduction ratio for the rear line pressure and that malfunction has occurred within the proportioning valve.

To test the valve, install gauge set shown, in brake line between master cylinder and proportioning valve, and at output end of proportioning valve and brake line as shown. Be sure all joints are fluid tight.

Have a helper exert pressure on brake pedal (holding pressure). Obtain a reading on master cylinder output of approximately 700 psi. While pressure is being held as above, reading on valve outlet should be 550 - 610 psi. If the pressure readings do not meet these specifications, the valve should be removed and a new valve installed.