Brakes (Calipers): Description and operation

The fluid in the chambers behind the plungers is unaffected by any movement of the plungers and can flow unrestricted between chamber and reservoir both before and during brake application.

When the brake pedal is released, the primary and secondary springs force their respective pistons back down the bore of the cylinder. As the plungers contact the stop pins the primary and secondary centre valves are opened allowing fluid to circulate unrestricted between the two hydraulic circuits and the fluid reservoir.

The movement of fluid during brake application/release is compensated for by fluid from the separate reservoirs within the supply tank moving through the feed holes in the cylinder. Conversely the final movement of the plungers causes any surplus fluid to move through the cut-off holes in the fluid reservoirs.

Should a failure occur in one system, the remaining system will still operate effectively, although the brake pedal travel will increase.

Calipers

Pressure at the caliper forces the caliper piston against the inner brake pad and, in turn, against the disc. The caliper body reacts and slides on the guide pins to bring the outer pad into contact with the disc.

ABS Operation

The ABS5E system is an ECU controlled system deriving its input from four speed sensors mounted at the wheels which allow it to control the hydraulic modulator to prevent the wheels locking during braking.

The operation of the ABS system is completely dependent upon electronic signals. To prevent the system responding to any inaccurate signals, a built in safety circuit monitors all electric and electronic signals, including battery voltage. If an inaccurate signal or low battery voltage is detected, the ABS system will shut down immediately and the warning light on the fascia will be illuminated to inform the driver that the brake system will respond to pedal pressure in the same manner as a non ABS system.

Modulator operation

For information on the electrical operation of ABS5E, See ELECTRICAL REFERENCE LIBRARY, Description and Operation.

With the vehicle in motion, the ECU receives signals from the four wheel sensors. From these signals the ECU can determine the speed of the vehicle. This is the speed which the ECU uses as a reference when evaluating the deceleration of each wheel.

This reference speed is continually calculated, even during braking. If one or more of the wheels are decelerating faster than the others, indicating a wheel is near the point of locking, the anti-lock sequence will be initiated.

The hydraulic modulator has three operation phases:

1. Increase pressure phase: the ABS system is at rest and fluid pressure from the master cylinder is allowed to pass through the solenoid valves in the hydraulic modulator to operate the caliper on each wheel.
2. Maintain pressure phase: The caliper is isolated from the master cylinder preventing any pressure increase due to increased pedal pressure.
3. Decrease pressure phase: The caliper is connected to the return pump which pumps the fluid back to the master cylinder.

Brake pressure proportioning valve

The fluid lines for both hydraulic circuits are connected to the valve, but the circuit for the front brakes uses the valve purely for distribution.

Pressure to the rear brakes passes through a piston sleeve, past a poppet valve and out to the rear brakes. The same pressure is also felt on the top of the piston, forcing the piston against spring pressure, towards the valve centre until the piston sleeve contacts the poppet valve and forms a seal. Input pressure is now balanced with output pressure. As further input pressure is applied from the master cylinder, it overcomes output pressure being felt on the piston and forces the piston outwards slightly away from the poppet valve allowing pressure past the poppet valve until output pressure is again balanced with input pressure.