You guys are going to get sick of me asking questions on here. I am so excited about building and using things for this new venture. My question is about the air valves in a bellows. I had a piece of rubber roofing that I thought would make good valve material. It's lighter than normal rubber roofing and quite pliable. I glued a piece to some 1/8 plywood and when dried the plywood had warped. My question is how heavy can these valves be and still work properly. It's evident the thin plywood won't get a seal. What have you guys that have made bellows used? Thank you fellas for your wisdom . 

Consider going to a hobby shop and getting a better grade of thin plywood (aircraft grade, multi ply) rather than the cheap 3 ply available at big box stores and the like.  If you preseal the plywood with some form of paint or lacquer)  and use contact cement to glue on the rubber it should not warp.  Needless to say, don't stretch the rubber over the plywood when gluing on, just lay it on the surface.  Sealing the plywood will help in any case as you don't want it to warp over time due to humidity fluctuations.

Cover the ply wood and seat area with felt, stops the clack clack noise every time the valves operate.

I used a piece of Al sheet that was age hardened.  Glued felt to it, Contact cement IIRC it was 30 years ago now... Don't forget to make a "trapper" so the valve can't flip over.

dont use too light a material, my first set wanted to hang open (luan door skin) had to glue a washer to the back of all 5

Another easy source for the rubber is EDPM pond liner, usually able to be purchased by the foot (or less) at most home centers.  Makes experimenting pretty cheap.

I don't know what style of valve set-up you have done but I'm going to equate this to old metal working shapers and their clappers.  The clapper has to positively return to the flat position at the beginning of the stroke or really bad stuff happens.  Because of this, the center of mass has to be proportioned beyond the pivot point.  In the photo, ignoring the tool holder part, you can see the actual clapper has the hole offset slightly to one side so the mass is always correct to clap it "shut" under gravity.

Yes, it's a bit of a weird analogy and might not apply to the way you are doing things...just one way to get positive closure beyond relying on only the air pressure to shut valves.

A thrift store leather belt makes fine flap valves, glue, screws, staples any will secure it fine. 

Frosty The Lucky.