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You may be right and the valve spring preload is not enough to keep the seal between the valve and poppet.

It also correlates with the fact that at higher pressures (3200-3400 PSI) it leaks slower than at lower pressures.

The only 2 forces which maintain that seal are the air pressure and the valve spring.
The higher the pressure the larger the force applied against the poppet.

The first few hundred shots I did not see a leak.
The leak only started after shooting a few hundred shots.
I assume that the cap slowly unthreaded out.

Maybe I need to apply a small amount of loctite on the cap thread.
This is the most insigthful postg I wrote about
If removing valve run fingernail around sealing edge, my second replacement valve chipped after two magazines, much slower leak out of barrel than valve broken in half. Used drill and sandpaper to dress edge, then re-seated to brass fitting per Hajimoto's Gauntlet Upgrade video, still holding full pressure several months later. WM
I have looked at the valve through a magnifying glass.
There were some very small dents where the poppet seals.
Hopefully those dents are small enough and don't matter.
 
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This is the most insigthful postg I wrote about

I have looked at the valve through a magnifying glass.
There were some very small dents where the poppet seals.
Hopefully those dents are small enough and don't matter.
If you get a chance, check out Hajimoto's YouTube video of upgrades to the original Umarex Gauntlet. Learned procedure to properly burnish valve face to brass fitting after watching him do so to Gauntlet. In my case, after finding chip with fingernail, used similar trick with 120 grit sandpaper and drill. Hole in sandpaper allowed valve face against grit while metal valve rod inserted in drill from other side. One second worth of spin, light, even pressure, valve face to grit, no more nick. Hajimoto's burnish treatment next, no more leak. You may only need Hajimoto's burnish procedure, since no large chips. WM
 
If you get a chance, check out Hajimoto's YouTube video of upgrades to the original Umarex Gauntlet. Learned procedure to properly burnish valve face to brass fitting after watching him do so to Gauntlet. In my case, after finding chip with fingernail, used similar trick with 120 grit sandpaper and drill. Hole in sandpaper allowed valve face against grit while metal valve rod inserted in drill from other side. One second worth of spin, light, even pressure, valve face to grit, no more nick. Hajimoto's burnish treatment next, no more leak. You may only need Hajimoto's burnish procedure, since no large chips. WM

Burnishing can also be done dry, go slow, dont press hard, it has worked 9:10 times for me.
 
Burnishing can also be done dry, go slow, dont press hard, it has worked 9:10 times for me.
Hajimoto's procedure is dry, brass fitting to valve face with valve rod in drill. Ten seconds, IIRC, of slow spinning, light, steady pressure, as you state. Borrowed from this idea to dress nick out of valve face edge using sandpaper, then burnished. WM
 
I managed to finally get the valve and poppet to seal together.

It took about 4 hours and 5-6 times opening up the valve area.
I finally think I managed to burnish that valve to perfection.

I tried a rotary tool first 3 times, but on last try I did it by hand and that worked.

I also got the barrel out and cleaned it, and also got the trigger out and polished/lubed the trigger parts.