Brakes (Operation): Description and operation

Operation

Servo

Inlet manifold vacuum is transmitted through a hose and non-return valve to the servo. Inside the servo, this vacuum is felt on both sides of the diaphragm. When the brake pedal is pressed, the servo push rod opens a valve and allows atmospheric pressure to be drawn through the filter into the pedal side of the diaphragm. The pressure differential, acting on the diaphragm, increases the pressure being applied at the brake pedal and transmits it to the master cylinder through a push rod.

Master cylinder

When the foot brake is applied the primary plunger moves up the bore of the cylinder and the pressure created acts in conjunction with the primary spring to overcome the secondary spring, thus moving the secondary plunger up the bore of the cylinder simultaneously. Initial movement of both plungers causes them to be pushed off their stop pins, thus closing both primary and secondary centre valves. Further movement of the plungers pressurises the fluid which is directed into the two separate hydraulic circuits connected to the hydraulic modulator.

The primary circuit operates the front brakes and the secondary circuit operates the rear brakes.

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.

Modulator operation

Type: ABS 5.3

For information on the electrical operation of ABS 5.3 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.