Proportioning Valve

CAUTION: The use of aftermarket load leveling or load capacity increasing devices on this vehicle are prohibited. Using air shock absorbers or helper springs on this vehicle will cause the height sensing proportioning valve to inappropriately reduce the hydraulic pressure to the rear brakes. This inappropriate reduction in hydraulic pressure potentially could result in increased stopping distance of the vehicle.

When a premature rear wheel skid is obtained on a brake application, it may be an indication that the hydraulic pressure to the rear brakes is above the specified output from the proportioning valve. This condition indicates a possible malfunction of the height sensing proportioning valve, which will require testing to verify that it is properly controlling the hydraulic pressure allowed to the rear brakes. Premature rear wheel skid may also be caused by an incorrectly adjusted proportioning valve actuator assembly, or contaminated front or rear brake linings.
Prior to testing a proportioning valve for function, check that all tire pressures are correct. Also, ensure the front and rear brake linings are in satisfactory condition. It is also necessary to verify that the brakes shoe assemblies on a vehicle being tested, are either Original Equipment Manufacturers (OEM), or original replacement brake shoe assemblies meeting the OEM lining material specification. The vehicles brake system is not balanced for after market brake shoe assembly lining material.
If both front and rear brakes check OK, proceed to verify that the actuator assembly for the height sensing proportioning valve is adjusted correctly. See Hydraulic System, Proportioning Valve, Adjustment for the adjustment procedure. If the proportioning valve is adjusted correctly, proceed with the test procedure for the height sensing proportioning valve as follows:






1. Remove the actuator assembly adjustment nut. Remove the actuator assembly from the lever on the height sensing proportioning valve.






2. Remove the chassis brake tube coming from the junction block from the front of the height sensing proportioning valve. Remove the chassis brake tube going to the rear brakes from the back of the height sensing proportioning valve.






3. Install the required fittings from Pressure Test Fittings, Special Tool 6833 into the inlet port of the proportioning valve assembly, from which the chassis brake line was removed. Install the removed chassis brake line into the Pressure Test Fitting. Install the required fitting from Pressure Test Fittings, Special Tool 6833 into the required outlet port of the proportioning valve. Install the required fitting from Pressure Test Fittings, Special Tool 6833 into the required outlet port of the proportioning valve. Then install the removed chassis brake line into the Pressure Test Fitting.






4. Install a pressure gauge from Gauge Set, Special Tool C-4007-A into each pressure test fitting. Bleed air out of hose from pressure test fittings to pressure gauges, at the pressure gauges. Then bleed air out of the brake line being tested, at that rear wheel cylinder.
5. With the aid of a helper, apply pressure to the brake pedal until a pressure of 6895 kPa (1000 psi) is obtained on the proportioning valve inlet gauge. Then based on the type of brake system the vehicle is equipped with and the pressure specification shown on the following table, compare the pressure reading on the outlet gauge to the specification. If outlet pressure at the proportioning valve is not within specification when required inlet pressure is obtained, replace the proportioning valve.
6. Remove the pressure test fittings and pressure gauges from the proportioning valve.
7. Install the chassis brake lines in the correct ports of the proportioning valve.
8. Install the pressure test fittings and pressure gauges in the opposite inlet and outlet port of the height sensing proportioning valve. Repeat steps 4 and 5 for the other proportioning valve.
9. Remove the pressure test fittings and pressure gauges from the proportioning valve.
10. Install the chassis brake lines in the correct ports of the proportioning valve.
11. Install the actuator on the height sensing proportioning valve. Adjust the proportioning valve actuator. See Hydraulic System, Proportioning Valve, Adjustment for the adjustment procedure.
12. Bleed both rear hydraulic circuits at the rear brakes.
13. Road test vehicle.
Proportioning valve split point pressure conversion is 1 bar is equal to 14.5 psi.





PROPORTIONING VALVE IDENTIFICATION CHART