337 Oxygen Leak

I am a retired 747 captain and have been restoring a T337C for seven years. The restoration is nearing completion and I need to find and stop a slow high pressure oxygen leak. I have bubbled with soap solution all connections many times and found no leaks. Then I repeated the process on all connections with Leak-Tec, again found no leaks.

Does anyone know of a leak detector system or process (probably electronic) that might pick up this small oxygen leak? Or does anyone have any other suggestion on how I should proceed. I hate to install a new headliner without solving this problem first.

Thanks,
Jim Hatfield