The pressure tube and valve are all one piece that can be safely unscrewed from the action and the rest of the rifle while fully charged, I believe you will need to remove the stock to do so.. atleast on mine that is the case because it’s too tight against the tube to allow it to turn.
Once you have the tube by itself I would stick the valve end into a container of water and watch for bubbles. If you don’t see any after a few min you can simply flip the tube and stick the fill probe/gauge end in the water to see if it’s coming from that end.
Those two ends of the tube are the only places it’s possible for an air leak on this particular rifle.
Hope this helps
