Master cylinder assembly (Brakes)

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.

Installed on top of the cylinder is a brake fluid reservoir. 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 connects an earth to the instrument pack and illuminates the brake warning lamp.

Non 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 takes the recuperating seals past the cut-off holes in the cylinder wall. 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. This creates a depression in the pressure chambers, which causes the recuperation seals to momentarily collapse and allows fluid from the chambers behind the pistons to flow into the pressure chambers. Fluid from the reservoir flows through the feed holes to compensate for the loss of fluid from the chambers behind the pistons. The final return movement of the pistons exposes the cut-off holes and allows excess fluid in the pressure chambers to flow into the reservoir.

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.