Brake Proportioning/Combination Valve: Testing and Inspection
On a vehicle equipped with ABS, premature or excessive rear wheel ABS cycling may be an indication that the brake fluid pressure to the rear brakes is above the desired output.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 brake shoe assembly lining material is of satisfactory condition, and of the correct material specification, check for proper proportioning valve function using the following procedure.
Rear Brake Proportioning Valve And Brake Tube Locations:

1. Road test vehicle to be sure the vehicle is truly exhibiting a condition of excessive rear wheel ABS cycling. Since ABS cycles both rear brakes together both proportioning valves of the assembly must be tested. Use the following procedure to test the proportioning valve.
2. Remove one of the chassis brake lines coming from the ABS modulator, at
3. the proportioning valve assembly. Remove the hydraulic brake line going to one of the rear wheels of the vehicle from the proportioning valve.
4. Remove the 2 bolts attaching the proportioning valve to the frame rail.
CAUTION: When lowering the proportioning valve, care must be taken not to kink any of the chassis brake lines.
5. Carefully lower the proportioning valve for clearance to install the proportioning valve test fittings.
Proportioning Valve Test Fitting Installation:

6. Install the required fitting from Pressure Test Fittings, Special Tool 6833 or equivalent 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 or equivalent into the required outlet port of the proportioning valve. Install the required fitting from Pressure Test Fittings, Special Tool 6833 or equivalent into the required outlet port of the proportioning valve. Then install the removed chassis brake line into the Pressure Test Fitting.
Pressure Gauges Installed On Pressure Test Fittings:

7. Install a pressure gauge from Gauge Set, Special Tool C-4007-A or equivalent 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.
8. 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.
9. Repeat steps 2 through 7 for the other proportioning valve of the assembly.
CAUTION: When mounting the original or a replacement proportion valve on the frame rail of the vehicle install the mounting bolts in only the two forward holes of the mounting bracket.