"One-way" fuel line valve..?

(or will the gasoline evaporate anyway?)..
Yes, After a few days it will. I install electric fuel pumps along with the mechanical as a primer/booster using a manual switch.I don't drive my old car and truck daily. Before starting,hit the switch and allow the electric pump to prime and after it starts, several seconds later,turn the switch off. The mechanical pump will draw fuel thru the electric pump with the electric pump not running. Done this for years on many cars.Just my 2 cents.
P.S. Sounds like a very cool Demon you have !