Carb Fuel Evaporation?

Many of our old cars have the problem hard starting after sitting for long periods. My car is no exception. This subject has been discussed a number of times. The fuel disappears out of the fuel bowl for whatever reason. Some of the people on this forum have installed backup electric fuel pumps. I plan to do this but I found a problem with some of the pumps on the market. They don't all allow fuel flow when not turned on. I want a electric pump I can fill the carb and then run on the factory pump. What pumps have some of you guys used?