Brake Booster

The latest innovation in automobile brake systems is introduction of Brake Booster. A black canister looking circular, it’s housed at the back part of the engine case toward the driver, that is, between the pedal and the master cylinder. The basic principle of working is that using the engine vacuum, it multiplies and leverages the pedal force applied by the foot to the master cylinder for effective braking.



A brake booster has body, piston return spring, control valve mechanism, reaction mechanism and booster piston as its important parts, while the body itself is divided into two chambers-constant pressure chamber and variable pressure chamber-each separated by a diaphragm; and the pressure inside the variable pressure chamber is controlled by the control valve mechanism.

Brake BoosterWorking principle:

When you press your brake pedal, that force is transmitted to the brakes through a liquid. But then, the actual brakes require a force still bigger than what your leg applies for its effectiveness; this force multiplication which is generally 2-4 times the size of the diaphragm, is achieved in two different ways- leverage (mechanical advantage) and hydraulic force multiplication.

The details:

As you press the brake pedal, a shaft is pushed forward which is attached to the power brake booster which uses the engine vacuum to help give the pedal increased force, helping in turn to balance the high pressures of the braking system.  Together with your foot effort on the pedal, this shaft then moves to push a piston into the main brake cylinder, displacing as a consequence, brake liquid through the braking system.

This liquid enters forcefully into the brake cylinder through a rationing valve such that correct liquid quantities reach the respective wheels to act on the brake slave cylinders which clamp the pads on the brake rotors, slowing down your vehicle. For transmitting the force from the brakes to the tyres and then to the road, frictional force comes into play.

Need for using brake boosters:

Earlier, cars had mostly only drum brakes, not needing any power braking features as it already had some best advantages. However, when disc brakes became the order particularly on the front wheels, the need for power braking system using boosters was keenly felt for helping the drivers from strenuous exertion while applying brakes.

For constantly driving people and persons with poor physique, applying more pedal force frequently may become a nightmare resulting in safety issues; so the Brake Booster have become a blessing in disguise for easy braking for drivers applying just a little of pedal effort to stop vehicles when needed.

To work even under brake booster failure:

However should there be any vacuum failure due to any reason, you see, the brake has to still work. In such a failure condition, there will be no pressure difference between the constant pressure chamber and variable pressure chamber. When the brake is “OFF,” the piston comes back to the right by the action of the piston return spring.

Upon the brake pedal being pressed, the valve operating arm extends to the left pushing the reaction disc, booster push rod and the air valve. This action results in the master cylinder piston applying the braking force on the braking system, still making the braking system work, and stopping the vehicle.

Tests for functioning of brake booster:

A test called “Air tightness test under load” for confirming its working condition is described here.  When the engine is running, press the brake pedal and then stop the engine, and wait for some 30 seconds.  If no change is observed in the position of the brake pedal, it indicates healthy functioning of the brake booster.

Advantages of brake booster:

The chief function of a brake booster is to multiply the force given on the pedal to the master brake cylinder by making use of engine vacuum and pressure. Thus a brake booster cares for cars making brake working great. In the absence of brake booster, slowing down of your car may require enormous amount of effort resulting in physical strain and fatigue for the driver.