I bought a plastic gas tank for my sons 1988 300 4x4. For some reason, I can't get the gas cap to stop leaking. It seems when the tank is full it leaks fuel from the gas cap the worst obviously. I put a vent tube on the cap and it continues to leak from the underside of the fuel cap. I even tried to make another gasket to put onto the gasket that came on cap. Any suggestions would greatly be helpful.