Thanks for the feedback guys. Good food for thought. If I were to abandon the air bumper idea and keep the vacuum actuation I wonder what type of bumper would stand up to the forces. I've considered standard closed cell foam, memory foam, pipe foam, and cloth. How thick were your O-rings? I'm using ...

Yes, except it's a 1:1 ratio, atmospheric instead of chamber pressure pushes the piston back, pilot vacuum instead of back pressure, and the air cushion bumper. My main concern is with the air cushion and the possible limited-check valve. If it stays open too long the piston will slam into the rear ...

I'm making a PVC QDV in a 2" T, 2 x 2"-1.25" adapters, 1.25" ported pipe, and a 1" coupler for a piston. I found that the 1" coupler with two plugs on the ends makes a perfect piston when the plugs are not completely inserted. This leaves a gap on the ends just the right size for loose O-rings. Here...