Hi,
This is probably not your problem (or a likely problem for anyone), but a funny little problem I once found. I had a tiny knick in the line coming off of the fill valve, though I don't know how the knick got there. Anyway, I kept losing pressure in flight even though the plane would hold pressure on the ground for hours. As I was trouble-shooting, I accidentally found the problem when my palm pushed down on the line at the valve. In flight, the air would leak because any high-G maneuver (which was practically the whole flight with this bird) would push down just enough on the line (which had a bunch of metal tees right after the valve) to expose/open the knick. Check your lines!