XJ-6 L6-4.2L (1983)
Vacuum Brake Booster: Description and Operation
PURPOSE
A vacuum servo is fitted into the brake hydraulic circuit in series with the master cylinder, to provide assistance to the driver when the brake pedal is
depressed. This reduces the effort required by the driver to operate the brakes under all braking conditions.
OPERATION
The unit operates by vacuum obtained from the induction manifold and comprises basically a booster diaphragm and non-return valve. The servo unit
and hydraulic master cylinder are connected together so that the servo unit piston rod acts as the master cylinder pushrod. The driver's braking effort
is transmitted through another pushrod to the servo unit piston and its built in control system. The servo unit piston does not fit tightly into the
cylinder, but has a strong diaphragm to keep its edges in constant contact with the cylinder wall, so assuring an air tight seal between the two parts.
The forward chamber is held under vacuum conditions created in the inlet manifold of the engine and, during periods when the brake pedal is not in
use, the controls open a passage to the rear chamber so placing it under vacuum conditions as well. When the brake pedal is depressed, the vacuum
passage to the rear chamber is cut off and the chamber opened to atmospheric pressure. The consequent rush of air pushes the servo piston forward in
the vacuum chamber and operates the main pushrod to the master cylinder.
The controls are designed so that assistance is given under all conditions and, when the brakes are not required, vacuum in the rear chamber is
established when the brake pedal is released. All air from the atmosphere entering the rear chamber is passed through a small air filter.
