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INDEX

General Information
    Brake Warning Switch Operation
    Hold-Off Unit Operation
    Proportioning Unit Operation
Service Procedures
    Brake Warning Switch
        Checking The Brake Warning Switch Unit
    Hold-Off Valve
        Checking the Hold-Off Valve
    Proportioning Valve
        Testing The Proportioning Valve Unit
Valve Application Chart

GENERAL INFORMATION  (back to CONTENTS)

All models have a hydraulic system control valve in the brake system (Fig. 1). The valve is attached to the fender splash shield, near the master cylinder.

The control valve assembly combines a warning switch with a hold-off valve and a proportioning valve.

Hold-off and Proportioning valves are used because of different braking characteristics between disc and drum brakes.

There is only one valve assembly in each vehicle, see “Valve Application Chart”. During any service procedures identify valve assemblies by part number as well as hold off/cut in (PSI) and split point (PSI).

Brake Warning Switch Operation
The hydraulic system brake warning switch warns the vehicle operator that one of the hydraulic systems has failed. A failure in one part of the brake system does not result in failure of the entire hydraulic brake system. As an example, failure of the rear brake system will leave the front brake system still operative.

If a pressure loss occurs in one side of the dual brake system, the difference in pressure causes the piston to move to the failed side and latch in the position. This will cause the brake system warning light to come on and stay on after the pedal is released. After the system is repaired and bled (see “Bleeding Brake System” in service adjustments), a moderate application of the brake pedal will hydraulically re-center the piston and automatically turn off the warning light. THE VALVE SHOULD NOT BE DISASSEMBLED TO RESET THE PISTON.

Proportioning Unit Operation
The proportioning valve section operates by transmitting full input pressure to the rear brakes up to a certain point, called the split point, and beyond that point it reduces the amount of pressure increase to the rear brakes according to a certain ratio.

Thus, on light pedal applications, approximately equal brake pressure will be transmitted to the front and rear brakes, while at higher pressures, the pressure transmitted to the rear brakes will be lower than to the front brakes to prevent premature rear wheel lock-up and skid.

If hydraulic pressure is lost in the front brake system, rear brake system hydraulic pressure moves the brake warning switch piston and opens a bypass in the proportioning unit allowing full rear brake hydraulic pressure.

Hold-Off Unit Operation
The hold-off valve section holds off pressure to the front disc brakes to allow the rear drum brake shoes to overcome the return springs and begin to contact the drums. This valve keeps the output pressure to the front brakes in the 3 to 30 psi range until the hold-off pressure is reached (117 psi) and then blends back to give full output pressure to the front brakes under heavy brake applications. This feature helps to prevent the front brakes from locking under light pedal application when driving on icy surfaces. The holdoff valve has no effect on front brake pressure during hard stops.

VALVE APPLICATION CHART  (back to CONTENTS)

Car Line	Brake Type	Hydraulic Valve Type	Hold-Off Cut-In (PSI)	Split Point (PSI)/Slope	Identification Tag *
B, F, G, STD	Disc	W/P/H	117	300/.27	Black
B, F, G, Heavy Duty	Disc	W/P/H	117	220/.43	Blue
W/P/H Brake Warning, Proportioning, Hold-Off Valve Assembly * Colored tag under valve stem boot.

SERVICE PROCEDURES  (back to CONTENTS)

BRAKE WARNING SWITCH  (back to CONTENTS)

Checking The Brake Warning Switch Unit
The brake warning light will come on when the parking brake is applied with the ignition key turned “ON”. The same light will also illuminate should one of the two service brake systems fail.

To test the service brake warning system, raise the car on a hoist and open a wheel cylinder bleeder while a helper depresses the brake pedal and observes the warning light. If the light fails to light, inspect for a burned out bulb, disconnected socket, or a broken or disconnected wire at the switch. If the bulb is not burned out and the wire continuity is uninterrupted, check the service brake warning switch operation with a test lamp between the switch terminal and a voltage source.

If light still fails to light, disconnect the brake tubes from the valve assembly and install a new valve assembly. If a new valve is installed, bleed the system. The warning switch is not serviced separately. Do not remove the switch or attempt to repair.

After repairing and bleeding the brake system (see “Bleeding Brake System” in service adjustments), applying the brakes with moderate force will hydraulically recenter the piston and automatically turn off the warning light. DO NOT DISASSEMBLE TO RESET THE PISTON.

HOLD-OFF VALVE  (back to CONTENTS)

Checking the Hold-Off Valve
A visual check will show that the valve stem extends slightly when the brakes are applied and retracts when the brakes are released.

In case of a hold-off valve malfunction, remove the valve and install a new combination valve assembly.

CAUTION: SEE “BLEEDING BRAKE SYSTEM” IN “SERVICE ADJUSTMENTS” THIS GROUP, BEFORE ANY ATTEMPT TO BLEED THE HYDRAULIC SYSTEM ON VEHICLES EQUIPPED WITH HOLD-OFF VALVES.

PROPORTIONING VALVE  (back to CONTENTS)

Testing the Proportioning Valve Unit
When a premature rear wheel slide is obtained on brake application, it could be an indication that the fluid pressure to the rear brakes is above the reduction ratio for the rear line pressure and that a malfunction has occurred within the proportioning valve unit which should be tested.

To test the proportioning valve, proceed as follows: During testing, leave front brake lines connected to the valve.

(1) Install one gauge and “T” of set C-4007A between brake line from master cylinder secondary port and brake valve assembly (Fig. 2).
(2) Install the second gauge of set C-4007A to the rear brake outlet port between valve assembly and the rear brake line. (An adapter tube, made up locally of a 9/16 x 18 tube nut, short piece of brake tube and 3/8 x 24 tube nut will be required to connect the hose to the valve). Bleed the rear brake system.
(3) Have a helper exert pressure on the brake pedal (holding pressure) to get a reading on the valve inlet gauge and check the reading on the outlet gauge. If the inlet and outlet pressures do not agree with the values on the following chart, replace the valve.
(4) Valve assemblies for all models are shipped in the front failed position to improve bleeding. When installing a new valve, bleed the rear brakes first (see

Split Point (PSI)/Slope	Inlet Pressure From Master Cylinder	Outlet Pressure To Rear Brakes
220/.43	1000	500-600
300/.27	1000	430-550