I would definately take your tanks to a Paintball shop and have them check it out. Sounds like theirs a problem with the tank valves.
There is a guy in the B/S/T forum selling a new EVO he won. He says he'll sell for $50 less than the best new price you can find. Show him this link for a new EVO for $229.99 and buy his from him for $180.
Here is his sale: