So as expected I didn't manage to get one for free from a neighbor's friend . Anyway Free engine Free pump Free materials Now I'm trying to understand how to keep the project almost free. I should need a pilot valve which is enough expensive to stop the project. So I've already thought about modify a normal pressure switch to regulate the throttle by adding an extension to a leverage arm . Now my idea or better my question is , would be the air piston on the pressure switch enough strong to open a valve of an air gun? My idea is use it as a pressure discharge valve .