Combination (Proportioning) Valve

METERING VALVE
Metering valve operation can be checked visually with the aid of a helper. Observe the metering valve stem while a helper applies and releases the brakes. If the valve is operating correctly, the stem will extend slightly when the brakes are applied and retract when the brakes are released. If the valve is faulty, replace the entire combination valve as an assembly.

PRESSURE DIFFERENTIAL SWITCH
1. Have helper sit in drivers seat to apply brake pedal and observe red brake warning light.
2. Raise vehicle on hoist.
3. Connect bleed hose to a rear wheel cylinder and immerse hose end in container partially filled with brake fluid.
4. Have helper press and hold brake pedal to floor and observe warning light.
a) If warning light illuminates, switch is operating correctly.
b) If light fails to illuminate, check circuit fuse, bulb, and wiring. The parking brake switch can be used to aid in identifying whether or not the brake light bulb and fuse is functional. Repair or replace parts as necessary and test differential pressure switch operation again.
5. If warning light still does not illuminate, switch is faulty. Replace combination valve assembly, bleed brake system and verify proper switch and valve operation.

PROPORTIONING VALVE
Road test the vehicle and make several hard brake applications. If a premature rear wheel skid occurs this could be an indication the proportioning valve has malfunctioned.
1. Inspect and verify the rear brakes are clean, adjusted and operating properly.
2. Check the pressure in and out of the valve. The pressure should be equal up to the 250 psi split point.
3. Above 250 psi the output pressure should decrease on a43 slope.