Fit a party balloon over the muzzle, and another over the filler valve. Leave over night. If the barrel one blows up, its the firing valve; with the rifle empty and uncocked, blow through with a bottle to shift any dirt in the valve. If its the filler valve, try the same thing, or replace it. If neither balloons blow up, its the joint between the action and the pressure gauge; replace the two O-rings.

Gus