شیر یک‌طرفه بوستر ترمز معیوب

The one-way valve between the brake servo and its vacuum source, failed. It holds vacuum in the servo when manifold vacuum falls — under acceleration, or with the engine off — which is what gives several assisted brake applications after the engine stops.

Failed, the servo loses its reserve. The pedal goes hard after one or two applications with the engine off, and assistance fades under acceleration when the engine produces least vacuum. That is the dangerous moment, which is why the valve exists. It is also a vacuum leak when it fails open, admitting unmetered air and upsetting the idle — so a hard pedal alongside a rough idle is the pairing. Test it by hand: it should pass air one way and seal firmly the other.