I have the same problem in my FX

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There's quite a few orings on the impact yes but far from all are under pressure all the time to cause this.

Gauge xrings
Small oring at the end of valve pin
Orings on valve house
Two outer orings on small end of plenum
Inner oring on plenum that seals against the valve rod.
Regulator orings

Off the top of my head, I'd get some leak detector and check these areas.
 
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Nate, I never actually checked, bu
There's quite a few orings on the impact yes but far from all are under pressure all the time to cause this.

Gauge xrings
Small oring at the end of valve pin
Orings on valve house
Two outer orings on small end of plenum
Inner oring on plenum that seals against the valve rod.
Regulator orings

Off the top of my head, I'd get some leak detector and check these areas.
to add onto your list of o rings as a part of the air pressure, you have one under that tiny pan head screw on the right side behind the vent hole(it’s the one screw some people never swap out when doing a stainless fastener conversion), the bottle itself has 3 o rings, and this next area ive never verified, but it’s the two plugs at the front and one at the rear of the trigger block. The front is hidden by the bottles Dowty seal, and at the rear it’s hidden by the hand grip. It’s the tunnel that FX drills thru, that is the pathway of air pressure to the regulator. Not looking at the parts diagram at the moment, but I could’ve sworn there were o rings under those screws.

To the OP- what kind of valve spring are you using in your gun? Factory or the lighter one that comes in the slug power kit? If you are using the lighter spring and DID NOT install that buffer rubber ball then your valve rod is over traveling, which will eat up that tiny valve rod o ring if it’s even still in there.

Drape a piece of toilet paper or light tissue over the cheek rest and have the sides cover the mag slot area. Do a dry fire. If that tissue blows up on the sides then that means your valve rod o ring is KAPUT
 
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Also, check that o ring that goes on the outer groove of the front end cap beak of that power plenum. Some of the o rings that came with the pp kits did not stick out past the threads when you look closely at the threads. Either the groove was machined too deep or the o rings supplied were not thick enough, and that’s a guaranteed leaker especially if the rear block flexes from setting it on a rear rest, pressure downwards.