Auxiliary fuel tank and gravity vs electronic pump

JRscooby wrote:
I'm hard of understanding here. If you have a line from 1 tank down to another, the fuel will flow. And if the level of fuel is above the fill neck of the lower tank, or any vent, what will stop fuel from coming out the vent?

Mine has a float valve where it ties into the main tank fuel fill line. It doesn’t let any diesel in once the tank is full. I suppose it’s similar to the valve that stops water flow once your toilet tank is full.