Proportioning Valve
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NOTE:
For proportioning valve testing on Anti-Lock Brake System (ABS), see appropriate ANTI-LOCK article.
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.
The proportioning valve for all disc/drum applications, split point is 400 psi, slope is 0.34, inlet pressure is 1000 psi, and outlet pressure is 525-625 psi.
- If the left rear proportioning valve is suspect, disconnect the tube nut fitting at the master cylinder primary port (port closest to power brake booster). Install Adapter, Special Tool (8494-2), in its place on the master cylinder.
- If the right rear proportioning valve is suspect, disconnect the tube nut fitting at the master cylinder secondary port (port furthest from power brake booster). Install Adapter, Special Tool (8494-2), in its place on the master cylinder.
- Connect primary brake tube to Adapter.
- Install a Pressure Gauge, Special Tool (C-4007-A), to the Adapter.
- Tighten all tube nut fittings to 145 INCH lbs. (17 Nm) torque.
- Remove screw fastening the speed control servo to the upper radiator closure panel. See Fig 1.
- Remove screw fastening the washer filler tube to the upper radiator closure panel.
- Remove nut and screw securing transmission control module to vehicle. See Fig 2.
- Lift the transmission control module (with speed control servo attached) from its mount leaving its wiring harness attached. Move it off to the side toward the engine making sure not to strain the wires and speed control servo cable.
- Clean any debris away from the fittings on top of the junction block.
- Using the following figure, remove the chassis brake tube leading to either the left rear or right rear brake at the junction block. See Fig 3.
- If the left rear proportioning valve is suspect, install Adapter, Special Tool (8494-3), in its place on the junction block.
- If the right rear proportioning valve is suspect, install Adapter, Special Tool (8494-4), in its place on the junction block.
- Install a Pressure Gauge, Special Tool (C-4007-A), to the Adapter.
- Tighten all tube nut fittings to 145 INCH lbs. (17 N.m) torque.
- 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.
- 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 acceptable pressure. 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 the appropriate value, replace the junction block (with internal proportioning valves). See JUNCTION BLOCK under REMOVAL & INSTALLATION. If pressure is within specifications, do not replace proportioning valve and perform the following steps.
- Remove the Pressure Gauge and Adapter, Special Tool (8494-3 or 8494-4), from junction block.
- Reinstall the chassis brake tube to the junction block port. Tighten tube nut fitting to 145 INCH lbs. (17 N.m).
- Install the transmission control module (with speed control servo attached) in its normal position. See Fig 2. Install the nut and screw securing it in place.
- Install the transmission control module mounting nut and screw securing it in place. Tighten the screw to 45 INCH lbs. (6 N.m). Tighten the nut to 107 INCH lbs. (12 N.m) torque.
- Install the screw attaching the washer filler tube to the upper radiator closure panel.
- Install the screw attaching the speed control servo to the upper radiator closure panel.
- Remove the Pressure Gauge and Adapter from master cylinder.
- Install the brake tube to the master cylinder primary or secondary port. Tighten the tube nut to a torque of 145 INCH lbs. (17 N.m).
- Bleed the affected brake line.
- 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.