Brake Proportioning/Combination Valve: Testing and Inspection

PROPORTIONING VALVE

NOTE: Only vehicles without antilock brakes have proportioning valves. Vehicles with antilock brakes have Electronic Variable Brake Proportioning (EVBP) which is built into the ABS Integrated Control Unit (ICU).





If 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 (1, 3).

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.

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 (25 mm) 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. Remove the hydraulic brake tube from the proportioning valve controlling the rear wheel that has premature wheel skid. If the left rear wheel is suspect, remove that brake tube (4) from the proportioning valve in the primary brake port. If the right rear wheel is suspect, remove that brake tube (5) from the proportioning valve in the secondary brake port.




3. Remove the proportioning valve (1 or 3) from its port in the master cylinder (2).




4. Install Adapter, Special Tool 8644 (1) onto the proportioning valve (3) inlet.
5. Install Adapter, Special Tool 6833-1 (2) into the proportioning valve (3) outlet.
6. Install the proportioning valve (with tools) back into the open outlet port of the master cylinder. Tighten Adapter 8644 in the master cylinder to 27 Nm (20 ft. lbs.).
7. Install the hydraulic brake tube in the proportioning valve outlet. Tighten the tube nut to 17 Nm (145 in. lbs.).




8. Attach a Gauge (1), Special Tool C-4007-A, to each pressure adapter.




9. Remove the brake pedal holding tool.
10. Bleed any air out of the pressure gauge hoses at the pressure gauge. Brake Bleeding
11. With the aid of a helper, apply pressure to the brake pedal until the reading on the proportioning valve inlet gauge is at the target inlet pressure shown in the Brake Proportioning Valve Applications And Pressure Specifications table at the end of 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.
12. 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.




13. Reinstall the brake holding tool on the brake pedal.
14. Remove the testing tools from the vehicle and proportioning valve.




15. Lubricate the O-ring seal on the proportioning valve.If the original proportioning valve is being installed replace the O-ring seal. Do not use the old seal.
16. Install the proportioning valve (1 or 3) in the master cylinder (2) port and hand tighten until the proportioning valve is fully installed and its O-ring seal is seated into the master cylinder. Tighten the proportioning valve to 27 Nm (20 ft. lbs.).
17. Install the brake tube in the proportioning valve outlet. Tighten the tube nut to 17 Nm (145 in. lbs.).
18. Bleed the affected brake line. Brake Bleeding