Proportioning Valve
OVERVIEWIf premature rear wheel skid occurs on a hard brake application, it could be an indication that a malfunction has occurred with one of the proportioning valves.
One proportioning valve controls the right rear brake, and the other proportioning valve controls the left rear brake. Therefore, a road test to determine which rear brake skids first is essential.
Before testing the proportioning valve in question, inspect the rear brake linings for contamination or for replacement shoes not meeting the OEM brake lining material specifications.
The proportioning valve should always be tested prior to being replaced.
The in-line proportioning valves used on this vehicle require special pressure fittings to test the proportioning valves for proper proportioning valve function. The pressure fittings are installed before and after the proportioning valve being tested to verify proportioning valve is maintaining the required hydraulic pressure to the rear wheel brake which it controls.
The testing of proportioning valves for this vehicle, if equipped with ABS, can be found in ANTILOCK BRAKE SYSTEM.
PROPORTIONING VALVE TEST
The test procedure is the same for either rear proportioning valve. After road testing the vehicle to determine which wheel skids first, follow the procedure below for testing the suspect proportioning valve.
1. Using a brake pedal holding tool as shown, depress the brake pedal past its first one inch of travel and hold it in this position. This will isolate the master cylinder from the brake hydraulic system and will not allow the brake fluid to drain out of the master cylinder reservoir.
2. Use the figure shown to determine which proportioning valve needs to be tested.
3. Remove the hydraulic brake tube from the proportioning valve controlling the rear wheel of the vehicle that has premature wheel skid.
4. Remove the proportioning valve from its outlet port on the master cylinder.
CAUTION: Be sure the pressure test fittings being installed into master cylinder and proportioning valve, have the correct thread sizes needed.
5. Install the Brake Pressure Adapters, Special Tool 8644 and 6806-3 onto the proportioning valve.
6. Install the proportioning valve (with tools) back into the outlet port on the master cylinder.
7. Attach a Pressure Gauge, Special Tool C-4007-A, to each pressure adapter.
8. Remove the brake pedal holding tool. Bleed any air out of the pressure gauge hoses at the pressure gauge.
9. With the aid of a helper, apply pressure to the brake pedal until the reading on proportioning valve inlet gauge is at the target inlet pressure shown in the BRAKE PROPORTIONING VALVE APPLICATIONS AND PRESSURE SPECIFICATIONS table following this procedure. If the inlet gauge pressure overshoots its target pressure when the pedal is depressed, release the brake pedal, relieving the pressure in the system, before reapplying the pedal to reach the target pressure at the inlet gauge. This is necessary to get an accurate reading of the outlet pressure.
10. Once inlet pressure has been achieved, check the pressure reading on the proportioning valve outlet gauge. If the proportioning valve outlet pressure does not agree with value shown in the table, replace the proportioning valve. If proportioning valve is within pressure specifications, the valve is good and does not require replacement.
11. Reinstall the brake holding tool on the brake pedal and remove the test equipment from the vehicle.
12. Remove the tools from the proportioning valve.
13. Install the proportioning valve in the master cylinder and hand tighten until the proportioning valve is fully installed and its O-ring seal is seated into the master cylinder. Torque the proportioning valve to 40 Nm (30 ft. lbs.).
14. Install the brake tube on the proportioning valve. Torque the tube nut to 17 Nm (145 inch lbs.).
15. Bleed the affected brake line.
BRAKE PROPORTIONING VALVE APPLICATIONS AND PRESSURE SPECIFICATIONS