Quote Originally Posted by Hurst01 View Post
....I don't recall ever seeing one that would drop by itself when the engine starts.
To mimic this feature, put your foot lightly on the brake of your modern OEM vehicle and start the engine. Static, the diaphragm resists the light pressure, but as soon as vacuum is applied the pedal moves to match the light pedal pressure. A return spring will eliminate the static pressure, and you can limit the return position providing you verify the total pedal travel is enough for full brake action.