Recently I took my car out for a drive and had to
It sounds like the locking cap now has some wear in it and when it is closed it is not tight on the gasket, or your gasket is just worn or shrunk from age. I would suggest replacing the gasket that is between the base and the top of the locking cap. It is part number A-18360-G. If replacing the gasket does not cure the problem, try removing the gasket and putting a thin layer of silicone in the bottom of the gasket groove. Then reinstall the gasket but do not push down very hard on the gasket. Let it set up slightly on the silicone you just put in and let it dry before you try closing the cap. This should shim the gasket up slightly and will probably take care of your problem.