I understand exactly what you are saying about the way the piston and valve operate but I am still no wiser on how to fix it, it cocks and fires but almost no power but I have not stripped the piston to see whats in it,as far as I know it has to have some oil in it and a air gap but I could be wrong this thing is giving me brain ache,all I did initially was change the rotted seals for o rings that seemed to fit but the inner piston you can push in and you should not be able to so till I look inside the piston im still a bit stuck but im getting there.