Brake Proportioning/Combination Valve: Testing and Inspection

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

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 Fig. 4 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.