2004 Audi S4 Brake Booster Replacement Cost

The Check Engine Light may illuminate with a P0171 or P0174 code, which indicates that the fuel system is running too lean due to a vacuum leak in the brake booster.

When the brake booster fails, the braking pressure needed at the brake pedal increases dramatically, and, in some cases, the brakes may seem unresponsive. For vacuum boosters, failure will generally create a vacuum leak, and the vehicle may run roughly, become hard to start, misfire at engine idle, and exhibit poor fuel mileage. The check engine light may also illuminate, and OBD trouble codes for engine misfires, lean air/fuel mixture, and intake manifold pressure will be stored in the computer. If the brake system uses they hydro-boost system, there will likely be a power steering fluid leak on the ground, and the brakes will be very difficult or impossible to press.

It is not safe to operate any vehicle with any braking problem. If there's any sign that the brakes are not performing as originally intended, the vehicle should immediately be placed out of service and repaired.

Normally, a vacuum booster will last from 150,000 miles to the lifetime of the vehicle. In especially dry climates, dry rot may cause deterioration of the internal diaphragm, and require replacement.

Diagnosing a vacuum or hydro-boost system is typically simple. The servicing technician may only need to feel the brake pedal with the engine running, and, noting the difficulty, immediately suspect brake booster failure. The booster will then be tested manually, and confirmed faulty upon failure of those tests. For vacuum boost systems, this means sucking the air from the booster, and observing pressure drop. For hydraulic systems, the hydraulic pressure going into the booster must be measured. If the pressure is within specification, yet the brake booster does not function, the technician will understand there is an issue with the booster, and recommend replacement.

Excessive brake pedal stopping effort is not always caused by a defective brake booster. It can also be caused by engine vacuum leaks, glazed brake rotors and drums, very hard brake pads and shoes, or sticking, rusty disc brake calipers and drum brake components.

Once a diagnosis has been reached, the brake booster must be disconnected from the vehicle by removing the master cylinder, brake booster vacuum or hydraulic lines, and finally the brake booster.