I had a similar problem with my Bonnie but a shiny new chrome Triumph cover fixed me right up. And, you might check and insure that the cover lip, the edge of the reservoir and the edge of the rubber diaphragm are completely dry before you cinch it up

A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)