Brakes (Copy valve): Description and operation

COPY VALVE

A copy valve, mounted inside the hydraulic modulator unit, is used to ensure balanced operation of the rear brakes when an ABS sequence is triggered. During an ABS sequence, the copy valve operates by using pressure changes in the brake circuit controlled directly by the modulator valve.

These pressure changes cause a piston in the controlled circuit (A), to operate a mechanically linked (but hydraulically separate) piston assembly in the uncontrolled circuit (B).

During normal braking, pressure from the master cylinder (C) in controlled circuit (A) flows through the solenoid valve to the R.H. rear caliper (A4). In the uncontrolled circuit (B) pressure passes into chamber (B2) of the copy valve, past ball valve (B3) in piston (B1) to the LH rear brake caliper (B4).

In the event of a failure of an internal seal, fluid will enter the centre chamber and, when the brake pedal is applied, the increase in pressure will cause the vent plug to rupture and allow brake fluid to escape.

When the ECU detects via, the speed sensors, that one of the rear wheels is decelerating faster than the others, indicating a wheel is near the point of locking, a signal is sent to solenoid valve (D) moving it to the maintain pressure phase (2) preventing pressure being applied or relieved from the R.H. rear caliper (A4).

Note: Both pistons will only move sufficiently to create the maintain pressure position.

As the pedal pressure is increased slightly the increased pressure in chamber (A2) causes piston (A1) to move. This action moves piston (B1), via the push rod, sufficiently to close ball valve (B3) preventing fluid flow to the LH rear caliper (B4) maintaining the pressure in that circuit also.

If the signal from the wheel sensor indicates that the wheel is still near locking point, even with pressure maintained, the ECU moves solenoid valve (D) to position (3) allowing fluid from the caliper to pass to the return pump. As the fluid is pumped away the pressure is reduced in chamber (A3) allowing the pressure in chamber (A2) to move piston (A1) further. This movement is transmitted to piston (B1) via the push rod so causing a pressure drop in the circuit to the LH rear caliper (B4).

When the signal to the ECU from the wheel speed sensor indicates that the wheel is not being braked sufficiently, the ECU removes its signal to the solenoid valve returning solenoid valve (D) to position (1) allowing the pressure exerted on the brake pedal to be felt at the caliper.

This cycle continues until one of three conditions are met:

1. the brake pedal is released;
2. the vehicle has come to a rest;
3. the speed of the wheels is almost the same;

and is repeated at a rate of between 4 and 10 times a second, depending upon the condition of the road surface and can be detected by the driver as a pulsating movement of the brake pedal.

Though both rear wheels are sensed separately, if either wheel were to approach lock – up the solenoid valve will control circuit (A) pressure which in turn controls the pressure in circuit (B) via the copy valve.