It is a 2 line tank. I initially thought the vent was blocked, but when I fuel the tank when it is not in the plane, it seems fine. My next thought was that a line was getting folded or something when I put it in, so that is when I took out all of the foam around the tank to see what happened. I made sure the line was open and it still did the same thing. I think I am just going to get a new tank. Thanks for all the help.