I've never filled the tank all the way up, but from about 3/4's full it didn't leak. However, the rubber seal on the cap was mostly missing when my friends & I bought it, and it was rusty enough to plug the carb within 10-20 min of riding (even when not using the reserve). The cleaner seemed to do a pretty good job though; I don't feel much rust if I hook my fingers around under where that film used to be attached. Maybe before fixing it again I can try to knock out a couple dents in it, and weld on more appropriate fuel fittings that won't be choking the quite oversized fuel pump.
One other question... What would you call the style of handle bar that's on the bike right now? I'd like to get one thats a bit bigger & thicker walled to use so I can drill through it & route the grip wires inside. The one thats on it is from a bicycle and doesn't quite fit, plus seems too thin to try to drill & expect it to stay in one piece in use.