Audi S4 Brake Power Booster Unit Check Valve Replacement Cost

The power booster unit check valve is a component of the power brake system in a car, truck, or SUV. It makes sure that vacuum is consistently applied to the brake power booster and helps to maintain power assist momentarily should the vehicle lose engine power. The power booster and check valve are located near the brake master cylinder and fluid reservoir in the engine compartment.

To help a driver slow and stop a moving vehicle, the brake power booster provides a vacuum "assist" to the brake pedal. This vacuum assist essentially multiplies the force of the brake pedal as pressure is applied during braking. Braking is made possible with less effort from the driver. The power booster check valve allows vacuum created by the engine to suck air out of the power booster, while preventing air from entering. This allows the vacuum in the booster to remain consistent. The check valve also serves to maintain vacuum momentarily in the booster if the engine stalls, giving the driver an initial power assist before the brake system reverts from power braking to manual braking.

The primary symptom of a faulty power booster check valve is a brake pedal that is hard to press. If the check valve is not working properly or it is leaking, the power booster will not be able to hold a vacuum. As a result, the power assist to the braking system will diminish.

The brake power booster check valve is a simple component, but it is an important safety feature of a vehicle. Without a working check valve, the power booster will not operate properly and the brakes will be very hard to push. The larger the vehicle, the more difficult it is to stop, and the more the power assist is necessary. While the brakes will work manually (without power assist), they will be very difficult to operate - especially at higher speeds. The vehicle should not be driven with a faulty check valve. If the check valve fails completely during driving, and the engine stalls, power assist will cease immediately and without warning. If the brake pedal is becoming hard to press, the vehicle should be inspected as soon as possible for a faulty check valve or failing power booster.

The brake power booster check valve has no regular maintenance schedule. It is possible for the check valve to last the lifetime of the vehicle and it is rarely inspected unless there is a problem with the brake system. It is, however, usually made of plastic and can fail. It should be checked if the brake pedal is becoming difficult to press.

To determine if the brake power booster check valve is bad, a technician will start the engine without pressing the brake pedal and allow it to run so that vacuum from the engine can be applied to the brake power booster. After two minutes, the technician will press and hold the brake pedal, turn off the engine, and continue to hold the pedal for thirty seconds. If the pedal remains where it is, the power booster is able to maintain a vacuum correctly. If, on the other hand, the pedal begins to rise, the booster is losing vacuum. At this point, the problem could be a faulty power booster or a bad check valve. Again with the engine running, the technician can spray brake cleaner or a soap and water solution around the check valve gasket to check for a leak. The check valve can also be removed from the vehicle and tested by blowing air first in one direction through the valve, and then the other direction. Air should only be able to flow through the valve toward the power booster. If air flows both directions, the check valve is bad.

Replacement of the brake power booster check valve is straightforward. A technician will locate the check valve connected to the front of the power booster, disconnect the vacuum hose, and carefully pull the check valve from its gasket in the power booster. The gasket will also be removed and replaced. The new check valve is inserted into the power booster and the vacuum hose is attached. Once the repair is complete, the technician will test the brake system to make sure the problem has resolved.

Because a hard brake pedal can be caused by problems other than a bad power booster or check valve, RepairPal recommends having the entire brake system inspected by a repair professional to determine if other issues are at fault. Excessive brake pedal stopping effort can also be caused by engine vacuum leaks, glazed brake rotors and drums, very hard brake pads and shoes, bent or kinked brake lines, or sticking, rusty disc brake calipers and drum brake components.

The brake power booster check valve is designed to provide momentary power assist to the brake system if the engine stalls during driving. If the check valve does not work, and the engine stalls, power assist will cease immediately and can take the driver by surprise as the required force to stop the vehicle increases dramatically.

Replacement of the brake power booster check valve is not particularly difficult. Someone with intermediate DIY experience should have little problem completing the repair. However, correct diagnosis is important, especially since the brake system is the most important safety feature of a vehicle.