The standard combination valve is located in the engine compartment, directly under the master cylinder. It consists of 3 sections: the metering valve, the warning switch and the proportioning valve.
The metering section limits the pressure to the front disc brakes until a predetermined front input pressure is reached, enough to overcome the rear shoe retractor springs. Under 3 psi, there is no restriction of the inlet pressures; the pressures are allowed to equalize during the no brake period.
The proportioning section controls the outlet pressure to the rear brakes after a predetermined rear input pressure has been reached; this feature is provided for vehicles with light loads, to prevent rear wheel lock-up. The By-pass feature of this valve assures full system pressure to the rear brakes in the event of a front brake system malfunction. Also, full front pressure is retained if the rear system malfunctions.
The pressure differential warning switch is designed to constantly compare the front and the rear brake pressures; if one should malfunction, the warning light (on the dash) will turn On. The valve and switch are designed to lock On the warning position once the malfunction has occurred. The only way the light can be turned OFF is to repair the malfunction and apply a brake line force of 450 psi.
REMOVAL & INSTALLATION
See Figures 1 and 2
- Disconnect and plug the hydraulic lines from the combination valve to prevent the loss of brake fluid or dirt from entering the system.
- Disconnect the electrical connector from the combination valve.
- Remove the combination valve-to-bracket nuts and the combination valve from the vehicle.
The combination valve is not repairable and must be replaced as a complete assembly.To install:
- Use a new combination valve (if defective) and install.
- Torque the combination valve-to-bracket nuts to 37 ft. lbs. (49 Nm).
- Reconnect the electrical connector to the combination valve. Bleed the brake system.
Whenever work on the brake system is done, it is possible that the brake warning light will come on and refuse to go off when the work is finished. In this event, the switch must be centered.
- Raise and support the truck.
- Attach a bleeder hose to the rear brake bleed screw and immerse the other end of the hose in a jar of clean brake fluid.
- Be sure that the master cylinder is full.
- When bleeding the brakes, the pin in the end of the metering portion of the combination valve must be held in the open position (with the tool described in the brake bleeding section installed under the pin mounting bolt). Be sure to tighten the bolt after removing the tool.
- Turn the ignition key ON . Open the bleed screw while an assistant applies heavy pressure on the brake pedal. The warning lamp should light. Close the bleed screw before the helper releases the pedal.
- To reset the switch, apply heavy pressure to the pedal. This will apply hydraulic pressure to the switch which will re-center it.
- Repeat Step 5 for the front bleed screw.
- Turn the ignition OFF and lower the truck.
If the warning lamp does not light during Step 5, the switch is defective and must be replaced.