OK your tank valve open´s and gas up your valve. For blowback marker like your X7 it´s good to have enough Output pressure which is like CO2 got + a bit more cause of the more Expansion of CO2, around 700psi better more !!! Does your Valve look like here on the Pic ??? If it should work ! What happen when you screw your tank on, cock the hammer and pull the Trigger ??? (Please explain each move so we can fllow you) Could be from an O-Ring or an to tight screwed Frame, everthing !?!?!? If you got everthing it must be fixable !!!