Brakes (Servo): Description and 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 simultaneously. Initial movement of both pistons takes the recuperation seals past the supply ports from the reservoir. Further piston movement directs fluid pressure into the two separate hydraulic circuits.

Forward movement of the pistons also allows fluid to flow from the reservoir into chambers around the body of both pistons. This fluid can move freely between the master cylinder and the reservoir and is not affected by plunger movement, before or during brake application.

When the brake pedal is released, the primary and secondary springs force their respective pistons back down the bore at a faster rate than the fluid, thus creating a vacuum in front of the two recuperation seals. The recuperation seals collapse, allowing the fluid in the chambers around the body of the pistons to flow through the holes in the pistons, past the seals and into the chambers in front of the pistons. Fluid also flows from the reservoir to replace the fluid displaced from around the body of the pistons. As the vacuum is relieved, the continued rearward movement of the pistons forces excess fluid from around the body of the pistons back into the reservoir.

Twin pressure conscious reducing 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, fluid pressure to the front calipers is not reduced by the valve but transmitted to the calipers at the same level as the pressure leaving the master cylinder.

Pressure to the rear brake calipers passes through the piston sleeve, past the poppet valve and out to the rear calipers. 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.

This continual action reduces the pressure being applied at the rear calipers and minimises the risk of the rear wheels locking under severe braking conditions.

Because both halves of the valve are operating at the same pressure, each piston is opposed by the other. If a pressure drop is felt in one side of the valve, pressure build up in the other side of the valve causes the piston to move across unopposed, pushing the piston in the low pressure circuit against the pressure of springs.

In the event of a failure of an internal seal, fluid will enter the centre chamber and, when the brake pedal is applied, the increase in pressure will cause the vent plug to rupture and allow brake fluid to escape.

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.