While taking down the 392PA, I observed an interesting air flow pattern associated with the valve and o-ring seals.
The reason for the take down was some modifications I had in mind.
On the prior disassembly but a few days prior, I had coated the exterior of the valve body during assembly with generous quantities of petroleum jelly. Reasoning that such methods would aid assembly without o-ring seal damage, and any incompressible material in that area would aid in efficient transfer of the charge air to the pell.
I will say that on this day's disassembly I was surprised but not confounded by the pattern of petrolatum that showed.
The migration of material was decidedly towards the front most seal. Yes, towards the pump "head space". And so, cycled back through the inlet check valve and the pop-open valve it's self! Showing at last on the poppet and the transfer port. Full circle! I confess, a mess of petroleum jelly on all surfaces of the valve, inside and out. Corrosion control!
It is easy enough to visualize the pressure changes through pump up and discharge cycles. The hi pressure air effecting an even greater seal on the rear most valve o-ring. All the while a more "balanced" pressure differential was defeating the self energizing sealing at the front. Thus the migration.
Has this anything to do with enjoying air guns? Perhaps not.