"Sticky" valve

Has anybody note AF valve, with o-ring mod done or new valve, that guns power is going down and down, wihout no reason?

I did some o-ring mod last year and it worked fine until now it gave very low power. When it was ok, it shooted about 1200 fps/Beeman Kodiak now it was only some 800 fps,,,both full power set.

I think that o-ring lube has blew off when shooting and stem/o-ring friction seems to be too much that it’ll give you full power. I dissassemble valve (and gun) and only false I found was this o-ring binding, it was good shape by visual.

I think that NBR based o-ring isn’t the best material for that use, polyurethane might be better, but where to find right size?

If you are getting tank dump, this o-ring binding might be one reason, I think. When shooting, air behind o-ring (tank pressure) will press o-ring very hard against stem. And if there is no lube that might work like air powered brake. If it feel like some binding without pressure, it definately will have much bigger binding when shooting under pressure.
Applying lube over stem, under tophat without dissassy, won’t help very long.

Are those factory o-rings some other material than NBR?