Following:

http://www.airgunbbs.com/showthread....-down-on-power

I have tried more tinkering but the (IMHO) slightly 'too clever for its own good' design with a split PB ring is driving me crazy.

It's a fair idea in practise but there must be a reason 99.9% of air guns have never used it.

Unless it is sized absolutely 100% perfectly then it's never going to give the sealing of a more flexible synthetic or leather seal surely that would balloon out as it will be too tight and draggy or too loose and the split will allow leaks?

I thought of trying a nitrile O ring in the groove but the only ones to fit and actually enter the piston and seal were a pair of thin ones but after 100 shots or so they are not long for the bin...

Anyway, has anyone converted a PB Senior to a cup washer a la Junior, MK I etc?

The piston has a dimple at the seal end presumably from lathing so is it simply a matter of drilling/tapping here and fitting a MK I cup and support washer?

If anyone has done this or has ideas please share.

It would be handy if there was a more common seal/support (SMK??) that could be fitted rather than trying to locate and paying the premium for vintage parts.