Brake Caliper: Description and Operation

NOTE: To identify separate hydraulic circuits, they are referred to as HYDROSTATIC and POWER circuits.

^ Power circuit - Consists of rear calipers and servo cylinders, supplied by direct hydraulic power from the power valve.
^ Hydrostatic circuit - Consists of servo cylinders supplied by master cylinder pressure. The front calipers are supplied with direct hydraulic energy from the servo cylinders, comprising a combination of master cylinder pressure and direct hydraulic power.

OVERVIEW
Lucas Colette type calipers are used all around. The front disc brake calipers each house two pistons, and hydraulic pressure is supplied by a combination of power and hydrostatic circuits. The rear disc brake calipers each house one piston, and hydraulic pressure is supplied by the power circuit via a Pressure Conscious Reducing Valve (PCRV).

OPERATION
The operation of both front and rear calipers is in principle the same. The Colette type caliper consists of two main components, a carrier and a hydraulic body assembly. The carrier is bolted to the hub assembly. The hydraulic body slides on two greased guide pins housed in the carrier. The guide pins are sealed by the dust covers to avoid unequal sliding loads caused by dirt or corrosion.
When the foot brake is applied hydraulic pressure pushes the piston and, with it, the inboard pad on to the disc. The hydraulic body reacts and slides on the guide pins to bring the outboard pad into contact with the disc. The clamping force on both sides of the disc is then equal.
When hydraulic pressure is released, the piston seal retracts the piston a small amount. This allows the moving parts to relax sufficiently for the brake pads to remain in close proximity to the disc ready for the next brake application.