the lift pump is mechanical and it bolted to the right side of the engine. It has an arm sticking out of it that goes into the block and rides on the cam. The pump is only pumping when the engine it turning over or rotating. To my knowledge there is no "check valve" or any kind of device to stop fuel from flowing backwards. There are a couple problematic places in the fuel system. The most common instance would be the return line caps/orings and lines. When was the last time those were replaced?