:I really have no idea what to use for a diaphragm. I've
: often considered inner tube, and while it's a good piston pump seal, I
: think it would be too fragile for a diaphragm.

Funny you should use the word 'diaphragm', because that leads to an interesting solution. I've heard that women can use a silicone rubber sheet, called a dental dam, for a female condom. [Obviously the dental part means it was intended for a different orifice.]

O.K., sexual stuff aside, at work we use silicone rubber sheets for 'vac-wrap' machines -- machines that draw a vacuum under a part to apply pressure, often for pressing on fabrics or carpets that have pressure-sensitive adhesives. These bladders of silicone rubber conform to the shape of the part, and are cycled thousands of times before they tear. The stuff we use is about 1-1.5mm thick or so. You should be able to get it thru industrial suppliers like Granger or thru medical supply stores like White & White.

Dean

(people will be wondering about me after this post...)