POWERMASTER 760 VINTAGE 1974 AIR LEAK

Hi folks. Does anyone here have any viable ...doable fix for the inherent air leak from where the brass bolt enters the breech on a 1974 or similar model powermaster 760?

This leak is present on every powermaster of that era's design I've come across. Crosman fixed it by wrapping the breech bolt in rubber and that stopped the air from leaking when the gun was fired on the later versions of the 760.

A repair would be nice since the repairs I've tried however short lived (one or two rounds), actually increase the muzzle velocity plus/minus 25 FPS, which is really desirable.

If anyone out there has tackled this particular problem successfully I'd really like to know how...please!

Thanks for looking.

Bruce J (Biznawich)