Talon Tank Trouble

Tried pumping up my Talon SS tank with my Hill hand pump after the gun had rested all winter. It didn’t seem to be pumping up so I removed the tank from the pump and as I did so the tank vented all it’s air – it didn’t seem to have much air in it at the time it vented. I tried pumping it up again but it won’t hold air. I saw somewhere that the tank has to have a reasonable pressure in it to stop the valve from leaking but I don’t necessarily believe that. Any suggestions on what to do to get the tank pumped up and holding air? Thanks.