Is it safe to remove the air cylinder then fire the hammer to see if the air puff is the same ? Will the hammer slam into the end of the chamber and cause any damage ?
If the puff if air us the same then it's being pushed out by the hammer if it's gone or reduced then some or all has to be leaking past the valve stem.
Trouble is how to create a seal here without affecting the vital free movement if the valve?
I think o ring or lip seal would do that . A labyrinth type arrangement would be complicated