Hi , you could try buying a new gas cap and see if that fixes it. I read a forum message a while back where the guy replaced a bunch of parts trying to track down a pressure leak, even the gas tanks! It turned out to be a small rubber washer in a connection on a fuel line just off the gas tank. Hard to say if there is any way to check these fittings, but it might be worth a visual check. I'll post some info if I can find that message. ~Gerald