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