Fuel issues

[71 customcamper]

I just bought my first squarebody. It's an 86 one ton. I replaced the carb and fuel pump along with the fuel tank valve. The fuel bleeds off within 5 min of turning off the truck is there anyway to fix that or something that I'm missing? I live in California and I'm not really sure where to go to ask but do you have to keep the return lines to the tanks functional for emissions purposes? Because I'd like to convert to a electric fuel pump and get rid of that valve and just use one tank, but I'm not sure if I should get the one with the return line built in or the one with out the return line. Thanks for all the help in advance.