Power-assisted brakes are a common feature in modern vehicles, designed to ease the physical effort required to engage the braking system. Typically, this power assistance is sourced from the difference in pressure between the partial vacuum within the vehicle’s inlet manifold and the external atmospheric air.

The servo unit responsible for providing this power assistance is connected via a pipe to the inlet manifold. In a direct-acting system, the servo is positioned between the brake pedal and the master cylinder. When you press the brake pedal, it pushes a rod that subsequently moves the piston within the master cylinder. Additionally, the brake pedal operates a series of air valves connected to a large rubber diaphragm attached to the master cylinder piston.

When the brakes are not engaged, both sides of this diaphragm are exposed to the manifold’s vacuum. Pressing the brake pedal changes this setup: it shuts the valve connecting the rear side of the diaphragm to the manifold and opens a valve allowing external air to enter. The external air’s higher pressure forces the diaphragm forward, which in turn pushes on the master cylinder piston, thereby enhancing the braking force.

If you maintain the pedal pressure without increasing it, no additional air enters, so the braking pressure stays constant. Releasing the pedal reopens the space behind the diaphragm to the manifold, reducing the pressure and allowing the diaphragm to retract.

Should the vacuum supply fail—such as when the engine stops—the brakes remain functional due to a mechanical link between the pedal and the master cylinder. However, greater force is then required to operate the brakes. Some vehicles feature an indirect-acting servo, positioned within the hydraulic lines between the master cylinder and the brakes. This type of unit can be installed anywhere in the engine compartment, not necessarily directly in front of the pedal.

Like the direct-acting system, the indirect-acting servo uses manifold vacuum for boosting. Pressing the brake pedal generates hydraulic pressure from the master cylinder, opening a valve that activates the vacuum servo.