Brake Booster
WARNING: This page is about a different car, the 2006 Mercury Milan, 2006 Lincoln Zephyr, and 2006 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.
The power brake actuation system includes a vacuum assisted brake booster, vacuum check valve, vacuum hose and aspirator. The vacuum hose delivers engine vacuum to the vacuum check valve. The vacuum check valve is a one-way valve that allows engine supplied vacuum to the booster, and closes when engine vacuum is removed. The brake booster utilizes engine vacuum to reduce the amount of effort required at the brake pedal to actuate the foundation brakes.