Check Valve

WARNING: This page is about a different car, the 2004 Mercury Monterey and 2004 Ford Freestar. However, it is still accessible from the selected car via links, so may be relevant.

The function of the brake booster check valve is to allow manifold vacuum to enter the brake booster and prevent the escape of vacuum in case manifold vacuum is lost during sustained full throttle operation.

To test the function of the brake booster check valve:

Start and run the engine for at least 10 seconds.
Operate the brake pedal to check for power assist.
Disconnect the vacuum booster hose from the brake booster check valve. Do not remove the brake booster check valve from the brake booster.
There should be enough vacuum retained in the brake booster for at least one more power-assisted brake operation.