Well, it works... Sort of...
I had a couple of problems:
no 4mm balls ,plenty of 3,5,6,8,16 but not a 4 anywhere in the shed!
The 5mm in a 5.5mm bore restricts the gas flow considerably.
And, the carefully measured and calculated O ring is the wrong size for my 12.7 mm piston. The result is too much friction, and excessive hysteresis. I stretched a small one over the piston to try it but although it worked better, it leaked .
All in all though a promising start.
Sorry about the poor pics, camera has gone walk-about again.