How to Know If a Brake Booster Is Not Working

by KevinM
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It is very rare today to find an automobile that is not equipped with power assisted brakes. Power assisted brakes are more effective than non-assisted brakes, and they provide the driver with a more predictable brake feel while also greatly reducing braking effort. The downside is that the power braking system is quite complex, and a problem with any of the system components can impair braking. The power booster, sometimes called the vacuum booster, is one such component. Because it is driven by the vacuum created by the engine the booster is susceptible to vacuum leaks. There are three simple tests that drivers can easily perform to determine if the booster is working properly.

Step 1

Park the vehicle and allow the engine to run at idle for a minute or so. Push the brake pedal down and hold it. Turn the engine off and continue to hold the pedal down with light but steady pressure for about 30 seconds. The brake pedal position should not change during this time. If the pedal slowly moves up it indicates a leak in the constant pressure chamber of the booster.

Step 2

Park the vehicle and turn the engine off. Pump the brake pedal a few times to relieve any residual vacuum in the booster. Push the brake pedal down and hold it with light but steady force. Start the engine. The brake pedal should drop slightly, and the pedal should change from a high and hard feel to a normal feel. If it does not then this indicates that the booster might not be receiving vacuum from the engine, possibly due to a plugged vacuum hose, a vacuum leak or a defective check valve. It can also indicate a failure of the booster itself.

Step 3

Park the vehicle and allow the engine to run at idle for a minute or so. Turn the engine off and then immediately push the brake pedal down with light but steady pressure four or five times, waiting a few seconds between each push. The brake pedal should feel harder with each push, and it should come to a stop at a higher position each time. This is because each push of the brake pedal uses up a little more of the residual vacuum stored in the booster. If the pedal returns to the same high position each time it indicates that the brake power booster probably has a leak and cannot store the residual vacuum. It can also mean that the check valve is defective.

Step 4

Park the vehicle on level ground and turn off the engine. Open the hood and locate the vacuum booster. It is a dome-shaped assembly normally mounted to the firewall at the rear driver's side of the engine compartment. Locate the flexible vacuum line coming in to the booster. Use pliers or a screwdriver to remove any retaining clips and then slip the vacuum line off of the booster connection point. Have an assistant start the engine. You should hear air being sucked into the open end of the vacuum line. Place your finger over the end of the line. The vacuum should feel strong. With your finger blocking the end of the line have your assistant turn the engine off. The vacuum should remain strong for at least one minute after the engine has stopped. For greater precision connect an engine vacuum tester to the vacuum line when doing these tests and record the engine vacuum level.

Step 5

Park the vehicle and turn off the engine. Open the driver's door and slide in under the dashboard. Look up under the dash where the brake pedal attaches to the pushrod. Now slowly depress the brake pedal with your hand until the pushrod just starts to move. A properly adjusted brake pedal should depress about 1/4 to 1/2 of an inch before the pushrod starts to move. (This distance is known as "free play.") If the pushrod moves immediately when the pedal is only slightly depressed then it might mean that the pushrod is engaging the brakes slightly all the time. This will cause loss of residual vacuum in the booster when the engine is shut off, which will affect the results of the above tests.

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