For the vents that aren't controlled by a ball valve, skip the pipe all together and just use magnets. We had one rust up on us and didn't realize it until we were lighting a drum @ 2 AM in the parking lot of the Topeka ExpoCenter. Needless to say, we weren't exactly thrilled. When I got home I cut them off w/ a Dremmel and replaced with fridge magnets. Works like a charm and don't have to worry about them rusting ever again.