Are you sure that it's not the air pressure escaping back past your blowgun nozzle. I don't hold constant pressure, just enough of a shot to be able to hear the clutch going on and then off again as I release the pressure.
If you have got a loss of pressure, from memory I'd say one of the sealing rings on the back of the drum could be damaged, but that's without the benefit of having a drum on front of me to look at.