With Antilock Brakes

CAUTION: Proportioning valves should never be disassembled.

Proportioning Valve:

If premature cycling of the rear brake ABS occurs on hard brake application, it could be an indication that a malfunction has occurred with one of the proportioning valves.

Proportioning Valve Location:

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 slides first is essential. Once the wheel which is skidding first is determined use the following procedure to diagnose the proportioning valve.

The test procedure is the same for both rear proportioning valves. After road testing the vehicle to determine which wheel skids first, follow the procedure below for testing the required proportioning valve.

Brake Pedal Holding Tool:

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. Remove the chassis brake tubes from the proportioning valve controlling the rear wheel of the vehicle which has premature wheel skid.

NOTE: Removal of the brake flex hose routing bracket is required to prevent bending the chassis brake line going to the rear brake. If the routing bracket is not removed the chassis brake line will need to be bent when installing the proportioning valve with the pressure test fittings installed.

3. Remove the brake flex hose routing bracket from the frame.

CAUTION: Be sure the pressure test fittings being installed into the proportioning valve have the correct thread sizes and flares for installation into the proportioning valve and for installation of the chassis brake tubes.

Pressure Test Fittings Installed On Proportioning Valve:

4. Install Pressure Test Fitting, Special Tool 6833-1 or equivalent in the inlet port of the proportioning valve. Install Pressure Test Fitting, Special Tool 8187-2 or equivalent in the outlet port of the proportioning valve. Tighten tube nuts to a torque of 17 Nm (145 inch lbs.).

Proportioning Valve With Pressure Test Fittings Installed:

5. Install the proportioning valve in the vehicle so that Pressure Test Fitting, Special Tool 8187-2 or equivalent in installed on the chassis brake tube going to the rear brake. Install the chassis brake tube coming from the HCU into Pressure Test Fitting, Special Tool 6833-1 or equivalent. Tighten tube nut to a torque of 17 Nm (145 inch lbs.).

Pressure Gauges Installed On Pressure Test Fittings:

6. Install a Pressure Gauge, Special Tool C-4007-A or equivalent into each pressure test fitting. Bleed air out of hose from pressure test fitting to pressure gauge, at pressure gauge to remove all trapped air.

Brake Proportioning Valve Applications And Pressure Specifications:

7. With the aid of a helper, apply pressure to the brake pedal until reading on proportioning valve inlet gauge, is at the pressure shown on the following chart. Then check the pressure reading on the proportioning valve outlet gauge. If proportioning valve outlet pressure does not agree with value shown on the following chart, when inlet pressure shown on chart is obtained, replace the proportioning valve. If proportioning valve is within pressure specifications do not replace proportioning valve.
8. Check rear wheel brake shoe linings for contamination or for replacement brake shoes not meeting OEM brake lining material specifications. These conditions can also be a possible cause for a premature rear wheel skid.
9. Install proportioning valve in chassis brake tube. Tighten the proportioning valve to a torque of 17 Nm (145 inch lbs.).
10. Bleed the affected brake line.