ORIGINAL: SkyDude
I ran into my first problem with it last month when the seal on the tank decided to fail. Make sure you check the screw on the tank cap and that it's sealed properly.
I will second this comment. The same thing happened to my Avistar's tank last week after about 100 or so flights. Check that rubber stopper for tightness. It seems fairly hard and if you can't get it tight it will need to be replaced. The Avistar is a great first AND second plane in one if you have an instructor.