What is Brake Booster and How it Works?


“Luxury must be comfortable, otherwise it is not luxury” very rightly said as when we talk about today’s automobile, luxury becomes the need of this modern era, so decreasing driver’s driving effort and ease of operating the automobile vehicle becomes the main focus for the designers and engineers which in turn gave rise to many small but effective innovative technologies in the field of modern automotive vehicles, so today we will discuss about one of these technologies that lies under your car’s hood  i.e. Brake booster , but what is it ?, where can you find this in your car ? , what does it do? Let’s just find out.

Brake Booster is a brake safety component that is coupled at intermediate position between brake pedal and brake master cylinder and works as force multiplication component that uses engine vacuum to multiply the force applied by the driver on the brake pedal before further sending it to the master cylinder which in turn provide effective braking as well as driver’s comfort in applying brakes.

What is Brake Booster

Image Source

It is a black diaphragm type component that is placed on the firewall of the engine’s compartment under the hood of the car just in front of the drivers legs.

Why We Need Brake Booster?

Brake Booster and Master cylinder

Image Source

As we have already discussed that brake booster is used intermediate of brake pedal and master cylinder but the question arises why? What is the need of using brake booster let’s just find out.

  • As we all know from article on master cylinder ,The brake force applied by the driver on the brake pedal is not enough to provide enough braking force to stop or de-accelerate the vehicle, so there is a need of multiplying the force initially and this multiplication of force is done by brake booster to cause an effective braking.
  • If we talk about the countries like India, where there is a lot of traffic problem which in turn requires continuous actuation of brakes or brake pedal and if we consider the cars without brake booster the pedal force required to actuate master cylinder is more which is a fatigue causing process for a driver, so we need brake booster for smooth actuation of brakes.
  • Brake booster is considered as the preventive braking measure taken to prevent driver and passengers as it decreases the chances of brake failure and also as it makes the braking more efficient and helps in case of panic braking.
  • In case of high speed super and hyper car which goes over 250 miles per hour the use of brake booster becomes very essential as they requires high braking force to stop or de-accelerate the vehicle at such a high speed.
  • In case of heavy vehicle as due to their heavy weight the brake force or frictional force required to stop or de-accelerate such vehicles is quite high, which can be fulfilled by the use of brake booster.

Also Read: 

Anti-lock Braking System (ABS) – Working Principle, Main Components with Advantages and Disadvantages

How Air Brake System Works in Automobile?

Drum Brakes vs Disc Brakes – Which is Better?

Constructional Details

Brake Booster being a small diaphragm shape components that is made up of parts that are-

1. Shafts or Booster Shafts 

They are the small circular shafts used as a linkages between pedal and master cylinder, 2 shafts are used inside a brake booster that are-

(i) Primary Shaft

It is the shaft attached to the brake pedal at driver’s side and is used to transmit brake pedal force to the diaphragm of the brake booster for the further pedal force multiplication.

  • This shaft opens and closes the atmospheric valve for entry and exit of fresh air inside brake booster for pushing the diaphragm.
(ii) Secondary shaft

This shaft is a mechanical linkage between diaphragm of the brake booster and brake master cylinder which is used to transmit the multiplied brake pedal force from the booster to the brake master cylinder.

2. Valves

As boosting brakes is a function of vacuum and atmospheric air which requires valve so in a brake booster 2 valves are used that are-

(i) Atmospheric valve

It is a valve placed at driver’s side of the brake booster and is used to regulate the entry and exit of atmospheric air with respect to the movement of brake pedal. i.e. actuation-valve opens up, released- valve closes.

(ii) Vacuum valve or one way valve

It is a one way valve placed at master cylinder’s side of the brake booster and is used to regulate and seize the vacuum inside the vacuum side of the brake booster.

Note – one way valve is a type of valve in which only one either entry or exit is permitted in case of brake booster (BB) exit of any air inside the vacuum side of BB is permitted but the entry of any air from outside is restricted.

  • This valve is connected to the engine’s inlet valve with the help of the hose pipe for using engine’s suction for maintaining vacuum inside the vacuum side of BB.

3. Springs

In a BB, 2 coil springs are used that are-

(i) Primary spring

It is the spring placed at the driver’s side of the BB and is used to return the primary shaft of BB to its initial position when the brake pedal is released.

  • It also regulates the opening and closing of the atmospheric valve.
(ii) Secondary spring

It is the spring used at the master cylinder side of the BB and is used to return the diaphragm of the BB(Brake Booster) at its original position after releasing of the brake pedal.

  •  Secondary spring is bigger in size than the primary spring.

4. Diaphragm

It is a part of a BB used as an intermediate membrane between the vacuum side and air side which separates the vacuum and air side and is actually responsible for pushing master cylinder with the help of secondary shaft.

  • It provides air tight separation between vacuum and air side.

5. Body

It is the outer body or outer casing inside which all the components mentioned above are placed.

  • It prevents the system from any foreign substance and also prevents the system from failure at the time of collision or accident.

Also Read:

Difference Between Flywheel and Governor?

What is Stirling Engine – Types, Main Parts, Working and Application?

How Engine Cooling System Works?


The working of brake booster is a function of vacuum and air that helps the booster to multiply brake pedal force before sending it to master cylinder let’s just see how does it happen-


  • When driver presses brake pedal in order to apply brakes the shaft attached to the brake pedal moves towards the brake booster.
  • Due to the movement of this shaft, the atmospheric valve opens up which in turn gives entry to the fresh air inside the air side of the booster.
  • This air entering assist the primary shaft to push the diaphragm inside the brake booster’s body due to the pressure difference between air side and vacuum side of the booster.
  • The vacuum valve at the vacuum side of the brake booster maintains the vacuum inside the vacuum side of the booster as it is attached to the engine’s suction valve which assists the brake booster in maintaining the vacuum during its suction stroke.
  • This vacuum or negative pressure developed assist the diaphragm to move which in turn multiplies the force sent by the driver.
  • Due to the movement of diaphragm the secondary shaft moves towards the master cylinder side which in turn pushes the master cylinder’s piston and finally braking occurs.


  • After the braking when driver puts his leg off the brake pedal i.e. releasing the pedal the returning spring attached to the secondary shaft in the vacuum side pushes the diaphragm away i.e. towards the driver’s side.
  • Due to this movement of the diaphragm the one way vacuum valve opens and pushes the left out air inside the vacuum chamber maintaining the vacuum inside the booster.
  • Due to this movement of diaphragm the returning spring attached to the primary shaft also pushes the primary shaft which in turn closes the atmospheric valve.
  • After the complete movement of primary shaft diaphragm, spring, valves, shafts and pedal all the components of the brake booster returns to their initial place and finally the releasing of brakes take place.

For Better Explanation About How Brake Booster Works Watch the Video Given Below:


Due to its safety and ease of actuation feature brake boosters are widely used in almost all the four wheel drive vehicles. some of them are-

Commercial cars like:

  • Maruti Suzuki swift
  • Hyundai i20

Heavy vehicles like

  • Mahindra alpha
  • Tata goi

Due to its safety features it is mandatory to use brake booster in all four wheel vehicle in many countries like America.

If this article enhances your knowledge about Brake Booster than don’t forget to like and share it on social networks like google+, Facebook etc. Thanks in Advance 🙂

Leave a Comment

Your email address will not be published. Required fields are marked *