IF you're using a tank and can't get a blast of air to seal it, you got a problem and it needs to come apart.
IF filling with a hand pump, it is possible that what you suspect is right...if you gave it a fast blast of pressure, it might seal and be fine.... but you can't easily generate a fast blast of air with a hand pump.
HAd the same situation with one rifle, no tank and the thought that a fast application of 800- 1000PSI would probably cure the problem (was a new valve stem, probably not quite perfectly mated to it's seat).
WHat I had was an old remote line for a co2 tank...and empty 3 1/2oz. co2 tank... and a fill probe already set up with a foster fitting. So I pumped up the 3 1/2oz. tank with air to 900PSI (which isn't any more dangerous that 900PSI of co2 as far as paintball tank strength is concerned), and hooked the remote line to the fill probe. In my case, it worked and the rifle held pressure. A 9oz paintball tank would work, just take a whole lot more pumping to get it up to 800-900PSI.
May not help your rifle if there is a bit of "funk" blocking the valve seat, or if that old valve seal finally just gave up, but if you've got the gear, it can be worth a try.
