Best way to dump air from 2 resevoirs?

[noname]

Well This is a really easy way to do it. Let me just draw something up real fast. Green is a bolt, blue are nuts, black are o-rings, and white are washers that hold the o-ring in place. You need a washer on both sides of the o-ring though. It only looks like one because the picture is really small.

[Mihlrad]

thats not easy at all, you need to find o rings and washers that make the perfect size.... i was considering expanding foam but i was unsure if it would make a perfect cylinder.

[carlbelcher]

Hot glue works just fine, but don't leave your cannon outside on a hot sunny day.

EDIT

You can also make your piston out of a PVC end cap, a 3" end cap fits great in a 4" pipe.