Chevy cut away chassis putting gas in

I have a 1986 Chevy truck with an aftermarket add on crossover fill for the two fuel tanks. A previous owner had installed it. Most pumps work OK but often I will run into a pump that will only work at a slow fill or if I turn the pump handle so it is nearly upside down the fuel will flow at a normal rate. It's awkward when you have to maintain the vacuum seal for the pumps with the vapor return boots like in California.