Understanding Power Brake Boosters

Power Brake Booster - Vacuum Assist

To understand the principle behind a brake booster, it is important to understand how a basic brake system works. No one would disagree that brakes are essential to the vehicle and safety of the passengers. Most of us haven’t placed much thought behind how it is that our cars stop.

In order for your car to stop, three scientific principles must come into play. Friction, leverage and hydraulics must all work in tandem to create the stopping power needed for a passenger vehicle.

The brake system is undoubtedly the most important system in your vehicle. To understand how the brakes work, it is important to understand the various components and technology behind the braking system.

The black, round canister mounted on the firewall at the back of the engine on the right driver’s side under the hood is the brake booster.


Since most cars now have disc brakes, power brakes are necessary to assist in multiplying the power exerted by the driver’s leg/foot. The force applied by the driver’s foot to the brake pedal is multiplied in the power booster. The power booster multiplies the force using a vacuum created by the engine.

The best method to determine if the brake booster is functioning properly is the following:

  • With the engine turned off, press and release the brake several times to assure there is no vacuum remaining in the booster
  • Using a fairly firm foot, hold down the brake pedal and turn on the engine
  • When the brake booster is functioning as it should, you will feel a slight give underfoot as the engine turns over.

The master cylinder and brake booster are replaced, removed and repaired as a single unit. If you have decided to do the installation yourself, there is one important item you should note. Often the master cylinder is filled with brake fluid before it is installed. Brake fluid is very damaging to paint so be sure to appropriately protect your car’s paint before filling the master cylinder with brake fluid any where near your vehicle.

If you are converting a SUV to larger tires, you will need to consider your braking system as well. Now would be the time to upgrade to a larger brake booster and master cylinder for the extra stopping power. Today’s larger and heavier vehicles should have upgrades on their boosters and master cylinders to safely drive on the streets. This is a complex conversion and will require special tools, do not attempt it if you are not fully confident in your abilities.

Since brakes are the single most important system in your vehicle, always consult with a specialist when upgrading anything to do with the tires, suspension or brakes in case you also need to upgrade your braking system. Also, anything changing dramatically the weight of your vehicle, such as trailers should be checked for necessary braking upgrades.

Understanding Brake Boosters - Diagram

Power Brakes – Vacuum Assist – Explained

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