Brake servo assembly (Brakes)

For all control functions, the ABS modulator regulates the hydraulic pressure to the brakes to control the speed of all four wheels, either individually or in axle pairs. Operation of the ABS modulator is controlled by the Self Levelling and Anti-lock Braking Systems (SLABS) ECU. The SLABS ECU also operates warning indications in the instrument pack to provide the driver with status information on each function.

Brake servo assembly

The brake servo assembly provides power assistance to reduce the pedal load when braking. If the brake servo assembly fails, the hydraulic system still functions but will require greater brake pedal effort due to the lack of vacuum assistance.

Two integral tie bolts attach the brake servo assembly to the pedal and bracket assembly on the engine bulkhead. The master cylinder assembly is attached to the forward ends of the tie bolts.

The brake servo assembly consists of a circular housing which contains two diaphragms, a central plate, a control valve assembly, input and output push rods and a filter. The input push rod is connected to the brake pedal. The output push rod locates in the primary piston of the master cylinder. A protective gaiter is installed on the control valve assembly where it extends from the rear of the housing. A non return valve, installed in a port in the front face of the housing, is connected to a vacuum line from the engine.

The control valve assembly consists of a valve body containing a valve, a piston, a valve spring and an input rod spring. The valve controls a vacuum port in the valve body. The piston controls an air inlet port between the valve and the piston. A reaction disc and a ratio disc separate the piston from the output push rod. A guide tube on the front of the valve body is attached to the front diaphragm and supported in a bush in the central plate. A return spring locates in the open end of the guide tube.

The two diaphragms and the central plate separate the interior of the housing into four sealed chambers. The chambers at the front of the diaphragms are connected together through fixed passages in the valve assembly. The chambers at the rear of the diaphragms are connected together through the interior of minor diaphragms on the tie bolts.

Brakes off With the brake pedal released, the piston in the control valve assembly positions the valve so that the vacuum port is open and the two pairs of chambers are connected together. When the engine is running air is evacuated through the vacuum line and non return valve, creating a partial vacuum in all four chambers. When the engine stops, the non return valve closes to maintain the partial vacuum and, on V8 models, prevent fuel vapour entering the brake servo.