I leave air in all my PCP guns.... sometimes to the max.
John,I try to keep my guns pretty close the full if I can, but my storage room has very moderate temperature swings. Just general information: If your room is going to be in the 80's or 90's, I'd give it some leeway for expansion.So maybe 3/4 full - that way you've got plenty of air for a moments notice, but not too much. PCP's should be overbuilt and ready to handle moderate excesses, but better safe than sorry.
I leave my Gen I FD-PCP fully charged all the time.. it hangs on the wall sometimes for a year at a time.
Had one of mine sit a year and half, full pressure (2,000 psi). No issues, worse to leave no pressure to my understanding.
The problem with guns stored long time at full pressure isn't the O-rings, these will do just fine,But the high pressure on the long run can get the poppet to take a little set ( not all of course )
Another way to put it is they are like valves on a car, you own one long enough you may have to give them some attention, some cars sooner than later FYI for the non train people lol, I could also adapt that to the air valve stem in a tire...
Leave mine full. HPA air is not going to change enough to be dangerous in living conditions. It’s not like CO2. Keep at least 1k to full no worries.
I store unregulated guns at or near max pressure. Regulated at or just below set pressure.
Close to the same…..for an air gun poppet would be the seal on the valve stem. Striker hits stem, poppet opens, air goes to pellet….air pressure and return spring quickly close the valve stem for the next shot.Like most of the answers, am also one to store a PCP near the bottom sweet spot pressure. Can’t store one rifle three different ways, but over all that seems to have worked out better for me.