Diagnosis And Testing - Proportioning Valve

Vehicles without Antilock Brakes have two proportioning valves. One proportioning valve controls the right rear brake, and the other proportioning valve controls the left rear brake. The proportioning valves are located in the junction block. Vehicle's with ABS do not have proportioning valves to test, they use Electronic Variable Brake Proportioning which is built into the Integrated Control Unit (ICU).

If premature wheel skid occurs on a hard brake application, it could be an indication that a malfunction has occurred with one of the two rear brake proportioning valves. Test the valve that controls the side of the vehicle on which the skid occurs. Both proportioning valves have the same pressure specifications and are tested in the same way.

1. Clean any debris away from the fittings on top of the junction block.
2. If the left rear proportioning valve is suspect, disconnect the tube nut fitting at the primary port of the junction block. See Fig 1. Install appropriate Adapter, Special Tool 6833, in its place on the junction block.
   Fig 1: Junction Block Fitting Identification

   Courtesy of DAIMLERCHRYSLER CORP.

   1 - FROM MASTER CYLINDER PRIMARY

   2 - FROM MASTER CYLINDER SECONDARY

   3 - TO LEFT FRONT BRAKE

   4 - TO RIGHT REAR BRAKE

   5 - TO LEFT REAR BRAKE

   6 - TO RIGHT FRONT BRAKE

3. If the right rear proportioning valve is suspect, disconnect the tube nut fitting at the secondary port of the junction block. See Fig 1. Install appropriate Adapter, Special Tool 6833, in its place on the junction block.
4. Connect previously removed brake tube to Adapter.
5. Install Pressure Gauge, Special Tool C-4007-A, to the Adapter.
6. Tighten all tube nut fittings to 17 N.m (145 in. lbs.) torque.
7. Remove the chassis brake tube leading to either the left rear or right rear brake at the junction block. See Fig 1.
8. If the left rear proportioning valve is suspect, install appropriate Adapter, Special Tool 6833, in the open port for the left rear chassis tube.
9. If the right rear proportioning valve is suspect, instal appropriate Adapter, Special Tool 6833, in the open port for the right rear chassis tube.
10. Connect previously removed brake tube to Adapter.
11. Install Pressure Gauge, Special Tool C-4007-A, to the Adapter.
12. Tighten all tube nut fittings to 17 N.m (145 in. lbs.) torque.
13. Bleed any air out of the system. This includes bleeding the air from the hose between the pressure test fitting and pressure gauge, which is done at the pressure gauge.
14. With the aid of a helper, apply pressure to the brake pedal until reading on proportioning valve inlet gauge (at master cylinder) is at the pressure shown on the chart at the end of this procedure. Then check the pressure reading on the proportioning valve outlet gauge (at junction block outlet to rear brake). If proportioning valve outlet pressure does not agree with value shown on the chart (once inlet pressure shown on chart is obtained), replace the junction block (with internal proportioning valves) (Refer to JUNCTION BLOCK ). If pressure is within specifications, do not replace proportioning valve and perform the following steps.
15. Remove the Pressure Gauges and Adapters from the junction block.
16. Reinstall the brake tubes to the junction block ports. Tighten tube nuts fitting to 17 N.m (145 in. lbs.) torque.
17. Bleed the affected brake lines. (Refer to BRAKES - BASE - STANDARD PROCEDURE) 
18. If no problem is found with the proportioning valves, check the rear wheel brake shoe linings for contamination or for replacement brake shoes not meeting OEM brake lining material specifications. These conditions can also cause premature rear wheel skid.