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Does your vehicle have a hard brake pedal? When we service vehicles with a hard brake pedal, we always start by looking at the brake booster. In this video, we discuss what a brake booster is and how it functions. We will also show you some tests and checks to determine if it is the cause of your hard brake pedal.

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Brake boosters, also called vacuum brake boosters, help improve braking performance and safety. When you hit the brake pedal, a shaft is pushed forward. This shaft is connected to the brake booster. The brake booster uses engine vacuum to give the brake pedal added force. This counteracts the high-pressure found in the brake system.

The brake booster itself contains a diaphragm separating the interior into two parts. Each part of the booster has a partial vacuum. When you hit the brake pedal, a valve inside the booster opens. This allows air into one side of the vacuum booster. The difference in pressure on this side pushes the rubber diaphragm to the other side of the booster. This helps push the piston inside the master cylinder, which displaces fluid throughout the brake system and applies the brakes.

The brake booster helps increase the force placed on the master cylinder when you hit the brakes. Without the brake booster, the amount of force it takes for you to stop your car using the brake pedal increases. Therefore, a malfunction in your brake booster can be the cause of a hard brake pedal.

Brake Booster Checks and Tests

Check Engine Vacuum

The first thing you want to do is make sure the engine can produce at least 18-inches of vacuum. Use a vacuum gauge to perform this test. Our vehicle did product 18-inches of vacuum.

Check-Valve Test

The check valve on the brake booster is holds a vacuum inside the brake booster even when the engine is off. This is a safety feature in the rare case that your engine fails. Without the engine, you lose your vacuum source. The vacuum stored in the booster- thanks to the check valve- will allow you to stop your vehicle safely.

An easy what to test the operation of the check valve is to disconnect the hose from the brake booster. If you hear a whooshing sound when you disconnect the hose, this is an indicator that the check valve is working.

Brake Booster Check

With the car turned off, pump the brake pedal four or five times until you get a hard brake pedal. Continue to hold the brake pedal down with moderate pressure and start the vehicle. The brake pedal should drop. If this brake pedal remains hard, there is a problem with the brake booster.

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