Hydraulic Control Assembly - Antilock Brakes: Description and Operation

Hydraulic Control Unit:

PURPOSE
The hydraulic unit either varies the brake fluid pressure in the wheel-brake cylinders during the regulating cycle, or keep it constant, whichever may be appropriate. The hydraulic pump assembly cannot by itself raise braking pressure, so that as a result the pressure which the brake master cylinder has built up cannot be exceeded.

CONSTRUCTION
The hydraulic unit consists primarily of three 3-way solenoid valves (one for each front wheel brake, one for the pair of rear wheels brakes), two reservoirs (one for each brake circuit) and the return-flow pump.

Pressure Build Up:

OPERATION
When brake application starts, the solenoid valves are de-energized, and the brake fluid can reach the master cylinder and the wheel brakes.

Pressure Hold:

As pressure builds up and the regulating action commences, the valve switches to its central position - the pressure-retention (hold) phase. This blocks the passage from the brake master cylinder to the wheel brakes, so that effective pressure at the wheel brakes cannot rise any higher.

Pressure Reduction:

In the subsequent pressure-reduction phase, the valve shifts to its third position. The connection between master cylinder and wheel brake remains interrupted, and brake fluid is now permitted to escape from the wheel brake cylinder to the reservoir. The return-flow pump, which starts as soon as the regulating action is initiated, delivers this fluid back to the master cylinder.

To build up pressure, the connection between brake master cylinder and wheel brake is re-opened. Since the master cylinder is still at increased pressure at this stage, the pressure in the wheel brake cylinder also builds up.