Here is the piston unit for testing. Mind you, this is a crude way of getting things to work. Not particularly taking a fancy to shaking the gun each end every time to get the piston out. Tried string, but is was getting hung up in the moving parts:
This is the piston seal with the cavity shown. The cavity volume is right at .0007cu.in./.011cc:
The next picture is the valve, again. This particular valve, though the same valve, has been altered at the piston side. If you can see it, there is a pronounced taper to match that of the conical shape inside the piston seal. There is a shelf the same with of the thickness around the edge of the piston to allow sealing contact:
The idea of tapering the valve face was to minimize losses between the o-rig and seal. Then, a well had to be created in the piston seal to allow clearance for the exhaust stem to pop and to act as air storage space. In addition, the well creates a more pliable piston seal wall.
"The majority of things in our lives are created by folks no smarter than the rest. Afterall, the world is comprised, and operated by C average people intellctually, academically, and morally. These people are often the great pioneers that set the precedent for what excellence should be."
...for how the resilience of your rubber cavity might (or not) be contributing to the energy budget of your SSP - just a vague feeling that it probably is.
And I have no idea where I got the misconception that you were using a stack of washers to space the piston. Just put it down to the slowness of a cold-blooded brain.