Another form of Servo brake system are the vacuum brakes where the suction from the engine inlet manifold is utilized for break application. There are two types of vacuum Servo brakes, both incorporating a piston or a diaphragm operating in a cylinder and providing provided with suitable linkage for brake application. A small vacuum Reservoir is also there to provide enough vacuum for several brake applications even after the engine has stopped. In the first type, both sides of the Piston are exposed to atmosphere when brakes are in the released position. For applying brakes, engine vacuum is applied on one side of the Piston, subjecting the same to differential pressure and thus operating the linkage. In the second type, however both sides of the Piston are subjected to engine vacuum in the brakes released position. To apply brakes, one side is exposed to atmosphere which provide the desired force on the Piston. This second system is called suspended vacuum system and is preferred over the first system, because this is comparatively more Rapid in action. A simplified diagram showing the suspended vacuum type of brake system is given in figure. Vacuum reservoir is connected through a non return valve to the inlet manifold between the carburator and the engine. Vaccum Reservoir is further connected to the servo cylinder on both sides of the Piston, on left side the connection is through the control unit, while on the right side it is direct connection. Control unit also contains a piston to which two valves are attached. The upper valve controls the connection between the atmosphere and left side of the Piston in the servo cylinder. The lower valve controls the connection between the vacuum reservoir and left side of the servo cylinder piston. The piston in the control unit itself is actuated by the brake pedal through the master cylinder as shown in figure.

When the brake pedal is free, upper valve in the control unit is closed and the lower one is open. Thus both sides of the Piston in the Servo cylinder are exposed to the engine vaccum. However, when the brake pedal is pressed to the apply the brakes, the pressure of the brake fluid pushes the Piston in the control unit up, thereby closing the lower valve there and opening the upper valve. Thus left side of the servo piston is exposed to atmospheric pressure, whereas vacuum acts on the right side. This causes the servo Piston to move to the right. This moment is utilized to apply the brakes in the wheels through some linkage which may be mechanical or hydraulic. Thus the force to be exerted by the driver for applying the brakes is considerably reduced and practically the whole of the braking effort is supplied by the engine vacuum.

A more commonly used sequence on Modern Automobiles is to have the Servo cylinder, called vacuum operated booster operated directly by the brake pedal through leverage. The booster unit actuates the pushrod to operate the master cylinder which is placed in front of the booster unit. Figure shows the layout of the Servo operated master vac brake system using drum type brakes on the rear wheels and disc brakes on the front wheels.

Typical mastervac installations are shown in figure. A cross section of the single diaphragm mastervac unit is depicted in figure. When the brake pedal is pressed, the valve operating rod of the unit operates the internal valve which admits the atmospheric pressure to right side of the diaphragm, which is normally having engine vaccum on both sides when the brakes are in the released position. The resulting differential pressure on the diaphragm is transmitted further to actuate the master cylinder through the pushrod. Thus in this system, force to actuate master cylinder to apply brakes is taken from the engine vaccum itself.

In the tandem diaphragm mastervac unit two vacuum suspended diagrams are used. The operation is similar to the single diaphragm described above. However, in this greater power assistance is achieved by the additional atmosphere / vacuum displacement.

Till now engine vacuum was the only source of power for vacuum assist brakes on cars. However, with the use of smaller engines and provision of emission controls, some vehicles do not have sufficient vacuum to operate the power brakes. In such cases, separate vacuum pumps are employed.