Hydraulic System: Description and Operation

NOTE: Anti-lock Brake System (ABS) is a standard feature, with Electronic Traction Control (ETC) optional.

GENERAL OPERATION
The brake system is hydraulically power assisted with an integrated, electronically controlled four channel ABS.
The use of a power assisted brake system means that, during brake application, additional hydraulic energy is provided by the hydraulic power unit. This hydraulic power unit consists of an electrically driven pump and an [1][2]accumulator which stores hydraulic energy in readiness for brake application. A pressure switch controls hydraulic pump operation to maintain fluid pressure in the accumulator.
The hydraulic system comprises two completely independent circuits, with a vertical (front/rear) split. The combined POWER and HYDROSTATIC CIRCUIT supplies the front calipers. The POWER CIRCUIT supplies the rear calipers.
The purpose of ABS is to prevent vehicle wheels locking during brake application, thus maintaining vehicle steerability and stability. This allows the vehicle to be steered while the brakes are applied, even under emergency conditions, and to avoid obstacles where there is sufficient space to redirect the vehicle.

UNIQUE HYDRAULIC COMPONENTS
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.

ABS Power Unit
The ABS power unit consists of an electrically driven pump, a pressure switch and an [1][2]accumulator.
The pressure switch incorporates three electro-mechanical switches: one for the pump, another, at a different pressure setting, to illuminate pressure warning light. The latter switch plus a third switch inform ECU of low pressure and that ABS function should cease while pressure remains low. The pump also incorporates a non-return valve and a pressure relief valve to protect the system.

NOTE: The pump and pressure switch are not serviceable; if failure occurs, a new unit must be fitted.