Ford Probe 1989-1992 Repair Guide

Brake System


The hydraulic brake system on the Probe is a diagonally split system with a dual master cylinder. The left front and right rear are on one brake circuit while the right front and left rear are on another brake circuit.

A dual proportioning valve regulates hydraulic pressure in the brake circuit. When the brake pedal is applied, full brake circuit pressure passes through the proportioning valve to the rear brake circuit until the valve's split point is reached. Above the split point, the proportioning valve begins to reduce hydraulic pressure to the rear brake circuit, creating a balanced braking condition between the front and rear wheels while maintaining balanced hydraulic pressure at each rear wheel. Double walled steel tubing extends from the master cylinder pressure fitting to the rear cylinders and front calipers. Flexible hoses make the final connections between the body mounted brake tubes and the suspension mounted brake assemblies.

When the brake pedal is depressed, it moves a piston mounted in the bottom of the master cylinder. The movement of this piston creates hydraulic pressure in the master cylinder. This pressure is carried to the wheel cylinders or calipers by the brake lines.

When the hydraulic pressure reaches the wheels, after the pedal has been depressed, it enters the wheel cylinders or calipers. Here it comes into contact with a piston or pistons. The hydraulic pressure causes the piston(s) to move, which moves the brake shoes or pads (disc brakes), causing them to come into contact with the drums or rotors (disc brakes). Friction between the brake shoes and the drums causes the car to slow down. There is a relationship between the amount of pressure that is applied to the brake pedal and the amount of force which moves the brake shoes against the drums. Therefore, the harder the brake pedal is depressed, the quicker the car should stop.

Since a hydraulic system operates on fluids, air is a natural enemy. Air in the system retards the passage of hydraulic pressure from the master cylinder to the wheels. Anytime a hydraulic component below the master cylinder is opened or removed, the system must be bled (of air) to ensure proper operation.

The wheel cylinders used with drum brakes are composed of a cylinder with a polished inside bore, which is mounted on the brake shoe backing plate, two boots, two pistons, two cups, a spring, and a bleeder screw. When hydraulic pressure enters the wheel cylinder, it contacts the two cylinder cups. The cups seal the cylinder and prevent fluid from leaking out. The hydraulic pressure forces the cups outward. The cups in turn force the pistons outward. The pistons contact the brake shoes and the hydraulic pressure in the wheel cylinders overcomes the pressure of the brake springs, causing the shoes to contact the brake drum. When the brake pedal is released, the brake shoe return springs pull the brake shoes away from the drum. This forces the pistons back toward the center of the wheel cylinder. Wheel cylinders can fail in two ways; they can leak or lock up. Leaking wheel cylinders are caused either by defective cups or irregularities in the wheel cylinder bore. Frozen wheel cylinders are caused by foreign matter getting into the cylinders and preventing the pistons from sliding freely.

The calipers used with the disc brakes contain a piston, piston seal, piston dust boot, and bleeder screw. When hydraulic pressure enters the caliper, the piston is forced outward causing the disc brake pad to come into contact with the rotor. When the brakes are applied, the piston seal, mounted on the caliper housing, becomes slightly distorted in the direction of the rotor. When the brakes are released, the piston seal moves back to its normal position and, at the same time, pulls the piston back away from the brake pad. This allows the brake pads to move away from the rotor. Calipers can fail in three ways, two of these being caused by defective piston seals. When a piston seal becomes worn, it can allow brake fluid to leak out to contaminate the pad and rotor. If a piston seal becomes weak, it can fail to pull the piston away from the brake shoe when the brakes are released, allowing the brake pad to drag on the rotor when the car is being driven. If foreign material enters the caliper housing, it can prevent the piston from sliding freely, causing the brakes to stick on the rotor.

Clean, high-quality brake fluid, meeting DOT 3 specs, is essential to the proper operation of the brake system. Always buy the highest quality brake fluid available. If the brake fluid should become contaminated, it should be drained and flushed, and the master cylinder filled with new fluid. Never reuse brake fluid. Any brake fluid that is removed from the brake system should be discarded.

Since the hydraulic system is sealed, there must be a leak somewhere in the system if the master cylinder is repeatedly low on fluid.

Click image to see an enlarged view

Fig. Fig.1Hydraulic brake system