Brake Booster 101: What Does It Do & Its Importance

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Brake boosters amplify the torque applied when you press the brake, meaning you can apply light pressure to the pedal and still feel confident your vehicle will slow down properly. Without a brake booster, you'd have to push hard on the brakes whenever you wanted to stop. 

How brake boosters work

Modern brake boosters have three components: a vacuum, air compressor and hydraulic unit.

When you press the brake pedal, air flows to the compressor and vacuum, managed by a valve. The air is forced to the hydraulic unit, where it moves a piston. The pressure from the brake pedal is amplified, increasing the pressure that goes through the brake pad, making the car stop more quickly. You can tap the brake lightly and still get substantial stopping power.

When you release the brake pedal, the vacuum booster opens, evening out the air pressure, so the power going through the brakes is released. 

The system is designed in such a way that it works even if the car's engine is off. It ensures that if the engine were to suddenly die, you'd still have some braking assistance to help you control the vehicle.

Brake boosters in electric vehicles
Electric vehicles take a different approach, using an all-electric system with electronic actuators to increase the force applied to the brake pads.

The end result feels the same to the driver. The driver can press lightly on the brake pedal and still feel confident enough braking force is generated to stop the car.

The components of a brake booster system

The brake pedal
The brake pedal is the part you interact most with as a driver, and it should feel responsive when you press on it. If it's spongy, it could mean you have air in the brake lines or they're leaking fluid. These are serious issues, so it's important to have your brakes checked if you notice they feel off.

When you press on the brake pedal, this pressure goes to the master cylinder, which starts the braking process.

The master cylinder
Master cylinders play a critical role in braking. The master cylinder contains a piston that translates the pressure you apply to the brakes into hydraulic pressure that goes down the brake lines.

Master cylinders can be single- or dual-reservoir systems. Single-reservoir master cylinders feed both the front and rear brakes from the same reservoir and the same brake line. Dual-reservoir systems have one reservoir to feed the brakes at the front and another for the back. This redundancy helps improve the safety of the vehicle.

Brake lines
The brake line is full of fluid, and it uses hydraulic pressure to move the brake cylinder and apply pressure to stop the car. Over time, the brake line could become damaged and start to leak. Even if you don't have a noticeable leak, some loss of fluid — and therefore loss of pressure — is normal and can lead to poor braking performance over time.

If you think you have a bad brake booster or your brakes aren't performing as well as they should, check your hydraulic fluid levels and the brake pads. Don't drive the vehicle if the brakes aren't working properly. Get it towed to a garage for a proper inspection.

Common issues with brake boosters

Brake boosters are generally quite reliable, but some things can go wrong with them. The most common issues relate to the vacuum hose and booster tube. If these become loose or the rubber tube/membrane of the booster is damaged, this could cause a leak, meaning the booster won't work properly. Fortunately, these components are relatively easy to replace.