Am not a gunsmith, only know basics. List of videos make by Ernest Rowe covering common leaks. Scroll down page.
https://airgun101.com/index.php/video-channels/fx-airguns

O rings are a law unto themselves, never managed to spot a bad one by looking at it, normally use leak spray to give me a headsup.

Does sound like a valve stem O ring......if its going up through the transfer port and down the barrel......or could be spring based. Am not a gunsmith....but you could email FX direct, they have a support section on the website.

UK has a new distributor who are now fully trained for this, maybe call them. Sportsman Gun Centre.
USA has FX USA who can help.

Unsure where you are in the world.

Don't panic, if you have a good description of what's going on and you have stripped the gun, am sure the part with the issue can be found and sent.

G

I had a similar problem on my Dreamline - mine was self inflicted, as I had dry fired it to death, to try and empty all the air out, I had ruined the valve seat. I could fill it to 230 and it was ok until I started firing it then it leaked about 10 bar every 2 hours.

so 1 new valve seat later and bingo, good as new.

Luckily, for me, the DL is mega simple to strip and reassemble - I doubt the impact is very hard to deal with.

O rings kits are cheap so it wouldn't hurt to swap the lot out and bang a new valve seat and see what happens - that would only cost about £30 and is bound to fix it.

yes I haven't a problem buying new seal kit and fitting but the valve seat are approx £45 for impact and after spending like £1800 few months ago id like this to last a little longer just hope ill get bits sent then I can have a speedy recovery if not back to sportsman gun centre she goes