Brake Fluid Accumulator: Description and Operation

HYDRAULIC ASSEMBLY COMPONENTS
Brake Booster
Mounted in same position as conventional brake master cylinder/servo unit, the brake booster contains the following components: fluid reservoir, power valve, master cylinder, isolating valve, ABS control valves and servo cylinders. It also contains the ETC solenoid control valves, when this option is fitted. The front face of the booster is marked ABS or ABS + ETC for identification.

NOTE: The brake booster unit is not a serviceable item, if internal failure occurs a new unit must be fitted. The fluid reservoir and its seals may be changed in the event of damage.

Fluid Reservoir
Mounted on top of the booster, the plastic reservoir is subdivided internally to provide separate capacity for brake fluid used in the hydrostatic and power circuits. A central tube incorporates a filter. A built in fluid level warning switch and a finer filter for the power circuit fluid are also incorporated. The fluid level warning switch is closed when there is sufficient fluid in reservoir.

NOTE: On vehicles equipped with manual transmission, the reservoir also supplies fluid to the clutch system.

WARNING: Extreme care must be taken when changing reservoir seals to avoid ingress of debris.

Master Cylinder
Operation of the master cylinder displaces a volume of brake fluid into the servo cylinders and increases fluid pressure. Piston movement inside the master cylinder will also activate the power valve.

Power Valve
The power valve is an extension of the master cylinder, it controls fluid pressure in the power circuit in direct proportion to pressure in the master cylinder. The power valve is of spool valve design.

Isolating Valve
The isolating valve consists of two solenoid valves controlling fluid inlet and outlet. Their function is to disconnect the master cylinder from the servo cylinders and to connect servo cylinders to reservoir return during ABS function.

ABS Solenoid Control Valves
Each pair, comprising inlet and outlet solenoid valves, controls ABS braking to each wheel. In response to signals from the ECU, the valves decrease, hold or increase brake pressure according to the need to retain wheel rotation and obtain optimum braking. The solenoid valves are designed to respond rapidly to ECU signals.

Servo Cylinders
Servo cylinders have five functions:
1. To provide combined energy from both hydrostatic and power circuit to brake calipers.
2. To provide 'brake feel' at the brake pedal.
3. To provide hydrostatic (master cylinder) braking through the servo cylinders to calipers in the event of no power circuit pressure to servo cylinders.
4. To provide braking from the power circuit and from hydrostatic fluid remaining in servo cylinder, in event of no hydrostatic circuit pressure from master cylinder.
5. To provide ABS control to the front calipers in response to pressure modulations in the power circuit.

Accumulator
The diaphragm type accumulator is fitted to the power unit. The accumulator is precharged with nitrogen at up to 80 bar, and its function is to store hydraulic energy.

NOTE: Accumulator replacement is possible if failure occurs. Correct disposal of old accumulators is essential.