Brocock/BRK Brck Bantam Sniper Leak

I have a .22 Bantam Sniper which is one of my favorite rifles, accurate, consistent and a joy to shoot.

Now, riddle me this....It has developed a slow leak. If I leave it with a goodly amount of air in it, say over reg pressure of 145bar, it will hold air forever (seemingly). But if I shoot it down to a lower pressure, say 100 bar or whatever, it will slowly over a couple days leak down to empty. It still shoots great, and consistent. Chrony shows it right where it has always been, so it's a conundrum.
I took the fill valve out and the oring looks great, it's the old style, not the new one with the check valve inside it, but the one that has a plain little rod with an oring on it. But it seems fine and putting a baloon over it, doesn't reveal a leak. So now I'm looking at the firing valve not sealing under light loads. One funny thing has happened twice that I remember. When shooing, it will fire normally and a small whoosh is heard. No noticeable air loss, and as I said, it's only happened maybe twice.
I haven't had the time or inclination to tear it down yet, but I'm leaning toward doing it in the next few weeks, if I can stop shooting it long enough to work on it :)
Any thoughts on how to proceed?