VACUUM SERVO BRAKES

When the vacuum is obtained from the manifold of the engine or a separately driven exhauster used to assist the braking effort the system is called vacuum servo-brakes.  The system consists of a vacuum reservoir connected through a non-return valve to the inlet mani­fold of the engine.  Two connections from vacuum reservoir, one on each side of the piston of the servo cylinder is provided; on left side the connection is through the control unit where as right side is connected directly.  The piston of the servo cylinder' is further connected to the piston of the boost cylinder or the brake linkage.  The control unit consists of a piston to which two valves are attached.  The lower valve controls the connection between the reservoir and the right side of the servo cylinder piston.  The upper valve controls the connection between the atmosphere and the left side of the servo cylinder piston.  The other side of the piston of the control unit is actuated by the pedal effort through a master cylinder.

When the brake pedal is at off position then the lower value is opened and the upper valve is closed.  Under this position the air from the atmosphere is disconnected and the vacuum from reservoir is created on both the sides of the piston of the servo cylinder.  When the brake pedal is depressed the brake fluid pushes the piston in the control unit.  This action closes the lower valve and opens the upper valve of the control unit, thereby -exposing the left side of the servo cylinder piston to atmospheric pressure and acting the vacuum on the right side.  This action of the vacuum in the right side of the servo piston moves it to the right thereby utilizing this movement through the mechanical or hydraulic means to the wheel cylinders and applying the brakes.  In this way the driver effort is utilized to control the positions of valves of the control unit and the vacuum effort is applied for braking through the booster unit.