Master Cylinder Assembly (Brakes)

To improve engine idle speed refinement, when the gearbox lever is in Park or Neutral, the solenoid valve is closed and the brake servo senses the vacuum in the inlet manifold via the venturi valve. When the gearbox lever is selected to positions other than Park or Neutral, the ECM energises the solenoid valve and air from the upstream side of the throttle body is fed to the venturi valve. The air flows through the venturi valve into the inlet manifold. As the air flows through the venturi in the venturi valve, it increases in velocity and decreases in pressure, resulting in an increase in the vacuum sensed by the brake servo. Depending on ambient conditions and engine speed, the vacuum enhancer system increases inlet manifold vacuum by a maximum of approximately 40%.

Master Cylinder Assembly

The master cylinder assembly produces hydraulic pressure to operate the brakes when the brake pedal is pressed. The assembly is attached to the front of the brake servo assembly, and comprises a cylinder containing two pistons in tandem. The rear piston produces pressure for the primary circuit and the front piston produces pressure for the secondary circuit.

The brake fluid reservoir is located either on top of the master cylinder assembly (LH drive vehicles) or above the master cylinder on a bracket on the bulkhead (RH drive vehicles). The reservoir is internally divided to provide an independent supply of fluid to each brake circuit, and so prevent a single fluid leak from disabling both primary and secondary brake circuits.

Should a failure occur in one circuit, the remaining circuit will still operate effectively, although brake pedal travel and vehicle braking distances will increase. If the fluid level in the reservoir is too low, the brake fluid level switch in the reservoir filler cap breaks a contact to the ABS ECU, which sends a CAN message to illuminate the brake warning lamp in the instrument pack.

ABS Master Cylinder When the brake pedal is pressed, the front push rod in the brake servo assembly pushes the primary piston along the cylinder bore. This produces pressure in the primary pressure chamber which, in conjunction with the primary spring, overcomes the secondary spring and simultaneously moves the secondary piston along the cylinder bore. The initial movement of the pistons, away from the piston stops, closes the primary and secondary centre valves. Further movement of the pistons then pressurizes the fluid in the primary and secondary pressure chambers, and thus the brake circuits. The fluid in the chambers, behind the pistons, is unaffected by movement of the pistons and can flow unrestricted through the feed holes between the chambers and the reservoir.

When the brake pedal is released, the primary and secondary springs push the pistons back down the bore of the cylinder. As the pistons contact the piston stops, the primary and secondary centre valves open, which allows fluid to circulate unrestricted between the two hydraulic circuits and the reservoir, through the centre valves, the chambers behind the pistons and the feed holes.

ABS

The ABS is a full time, four channel system that gives individual speed control of all four wheels to provide the vehicle with anti-lock braking (ABS), Electronic Traction Control (ETC.) Hill Descent Control (HDC) and Electronic Brake force Distribution (EBD) functions.

ABS Modulator The ABS modulator controls the supply of hydraulic pressure to the brakes in response to inputs from the ABS ECU. The modulator is attached by three mounting bushes to a bracket on the RH inner wing, and connected to the primary and secondary hydraulic circuits downstream of the master cylinder assembly. A multi-pin connector links the ABS modulator to the vehicle wiring.

Internal passages in the ABS modulator, separated into primary and secondary circuits, connect to the various components that control the supply of hydraulic pressure to the brakes. Separation valves and check valves control the flow through the internal circuits. Damper chambers and restrictors are included in each circuit to refine system operation. Inlet and outlet solenoid valves control the flow to the individual brakes. An accumulator is connected to each circuit to absorb flow surges. A common return pump is connected to both circuits to provide a pressure source and return fluid to the reservoir.