There are one-way valves available to insert into the pipe but as they only work "one way" (either on fuel expansion on sunny days or contraction on cool nights) you can end up with a vacuum and possible collapsed tank walls!!! My boat was kept under a full cover in the Cyprus sun and always smelt of fuel when I removed it, but never caused me a problem. Just ensure the vent exit is nowhere near a spark source and don't smoke when removing the cover.!!! Finally, don't worry about all the fuel venting away, it just doesn't happen!! I believe there may be a two-way valve available that requires a small over/underpressure before releasing but I am unable to find details., maybe someone else has the information.