Well there you have it. Pretty well explained. I know that air does leak backward through my vane pump when it's off. You can hear it. Without a check valve the water pushes its way into the line as Ted mentions. One of the reasons I went with the Thomas vane pump is that it is rated for 15 psi continuous. The higher it's rated the longer it should last, I hope! My water must be low in minerals and such since I've yet to have any buildup problems with my air stones or check valves.
