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heavier pedal applications the pressure transmitted to the rear will be lower than the front brakes. This will prevent premature rear wheel lock-up and skid. If hydraulic pressure is lost in one half of the diagonally split system, the operation of the proportioning valve in the remaining half is not effected.

ABS BRAKE PROPORTIONING VALVE OPERATION

On vehicles using the ABS braking system, screw in proportioning valves are used in place of the conventional differential pressure/proportioning valve (Fig. 3).

Each rear brake circuit has its own screw-in proportioning valve which is attached to the rear brake outlet ports of the hydraulic assembly. These valves limit brake pressure to the rear brakes after a certain pressure is reached. This improves front to rear brake balance during normal braking.

Fig. 3 ABS PROPORTIONING VALVE IDENTIFICA-

TION

Screw in proportioning valves can be identified by the numbers stamped on the body of the valve. The first digit represents the slope, the second digit represents the split (cut-in) point, and the arrow represents the flow direction of the valve. Be sure that the numbers listed on the replacement valve are the same as on the valve that is being removed.

See (Fig. 3) for detail of the valve identification.

BRAKES 5 - 27

HYDRAULIC SYSTEM SERVICE PROCEDURES

BRAKE WARNING SYSTEM

CHECKING BRAKE WARNING SWITCH UNIT

The Red 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 hydraulic systems fail.

To test the service brake warning system lamp. Raise the vehicle on a hoist and open a wheel cylinder bleeder while a helper depresses the brake pedal and observes the warning light.

CAUTION: Make sure air does not enter the hydraulic system during this test procedure. See bleeding without a pressure bleeder at the beginning of this section for master cylinder fluid level checking procedures.

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 known good ground. Be sure to fill master cylinder and bleed brake system after correction has been made, if necessary.

PROPORTIONING VALVES

TESTING PROPORTIONING VALVE UNIT

If premature rear wheel skid occurs on hard brake application, it could be an indication that a malfunction has occurred with the proportioning valve unit.

The proportioning valve is designed with two separate systems. One half controls the right rear brake, and the other half controls the left rear brake. Therefore, a road test to determine which rear brake slides first is essential.

RIGHT REAR WHEEL SLIDES FIRST

To test the proportioning valve when the right rear wheel slides first, leave the front brakes connected to the valve, proceed as follows:

(1)Install one gauge and (TEE) of set C-4007-A between the brake line from the master cylinder secondary port and the brake valve assembly.

(2)Install the second gauge of set C-4007-A to the right rear brake outlet port (Fig. 4). Using an adapter tube, made from a short piece of brake tube and (2) 3/8 x 24 tube nuts. Connect the hose to the valve. Bleed the hose and gauge.

(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