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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

Fig. 4 Tube Connection for Right Rear Skidding

gauge. If the inlet and outlet pressures do not agree with the values on the following chart, replace the valve.

LEFT REAR WHEEL SLIDES FIRST

To test the proportioning valve when the left 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 brake line from the master cylinder primary port and the brake valve assembly.

(2)Install the second gauge of set C-4007-A to the left rear brake outlet port (Fig. 5). An adapter tube, made up from a 7/16 x 24 tube nut, a short piece of brake tube and 3/8 x 24 tube nut. Will be required to connect the hose to the valve. Bleed the gauge and hose.

(3)Have a helper exert pressure on the brake pedal. Hold pressure steady 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.

Fig. 5 Tube Connection for Left Rear Skidding

TESTING ABS PROPORTIONING VALVES

All ABS components use an ISO type tubing flare. Use the correct adapters with ISO type tubing flares when installing gauges to test ABS proportioning valves.

(1)Install one gauge and (TEE) between the hydraulic assembly and the male end (Inlet) of the valve.

(2)Install the second gauge at the female end (Outlet) of the valve (Fig. 6).

Fig. 6 Tube Connections for ABS

(3)Have a helper exert pressure on the brake pedal (holding pressure) to get a reading on the valve inlet gauge.

(4)Check the reading on the outlet gauge. If the inlet and outlet pressures do not agree with the following chart, replace the valve. See (Fig. 3) for proportioning valve identification.

GENERAL INFORMATION

The single piston, floating caliper disc brake assembly (Figs. 1 and 2) consists of:

²The driving hub

²Braking disc (rotor)

²Caliper assembly

²Shoes and linings

²Adapter for mounting the caliper assembly to the steering knuckle

WARNING: THE PISTONS THAT ARE USED IN THE 2 DIFFERENT CALIPER ASSEMBLIES ARE UNIQUE TO THE CALIPER THEY ARE USED IN. THE DIMENSIONS OF THESE PISTONS ARE DIFFERENT, DO NOT INTERCHANGE THE CALIPER PISTONS. IMPROPER USE COULD CAUSE A COMPLETE FAILURE OF THE BRAKE SYSTEM.

The double pin Kelsey-Hayes Family Caliper, is mounted to the adapter using bushings, sleeves and 2 through bolts threaded into the adapter (Figs. 3 and 5). The adapter is then mounted to the steering knuckle using 2 attaching bolts.

The double pin Kelsey-Hayes Non-Family Caliper, is mounted directly to the steering knuckle of the vehicle using bushings, sleeves and 2 through bolts (Figs. 4

page

and 5). The adapter is not used on the vehicles equipped with the Non-Family caliper assembly.

Two machined abutments on the caliper mounting adapter or steering knuckle, (Figs. 3 and 4) position and align the caliper fore and aft. The guide pin bolts, sleeves and bushings control the float, side to side movement of the caliper. The piston seal, is designed to pull the piston back into the bore of the caliper when the brake pedal is released. This will help in maintaining proper brake shoe to rotor clearance (Fig. 6).

Vehicles equipped with the Kelsey-Hayes double pin family caliper. Have 1 anti-rattle clip attached to the top of the adapter (Fig. 1).

All of the braking force is taken up directly by the adapter or the steering knuckle depending on the type of caliper assembly the vehicle is equipped with.

The caliper is a one piece casting with the inboard side containing a single piston cylinder bore.

The front disc brake caliper phenolic piston is 60 mm (2.36 inch) in diameter.

A square cut rubber piston seal is located in a machined groove in the cylinder bore. This provides a hydraulic seal between the piston and the cylinder wall (Fig. 6).

A molded rubber dust boot installed in a groove in the cylinder bore and piston, keeps contamination from the cylinder wall and piston.

The boot mounts in the cylinder bore opening and in a groove in the piston (Fig. 6). This prevents contamination in the bore area.

As lining wears, master cylinder reservoir brake fluid level will go down. If brake fluid has been added to the reservoir, reservoir overflow may occur when the piston is pushed back into the new lining position.

Overflowing can be avoided in this case by removing a small amount of fluid from the master cylinder reservoir.

All vehicles, are equipped with an audible wear sensor on the outboard pad of the front disc brake assemblies. This sensor when emitting a sound signals that brake lining may need inspection and/or replacement.