Brendan M has the best check. There is some check valves in the fuel pump. If crud or wear has made them leak, then gravity will do it's thing and fuel will drain back to the lowest point. More noticeable with a low or almost empty tank. A filter between the tank and pump can help if it is dirt/crud. A rebuilt or new pump may be needed if the issue is wear. Some fuel lines are too close to hot spots on the engine(routing matters). Fuel in the line heats up and vaporizes. Pressure will then push that fuel back past the check valve in the pump. The results are the same. Removing the line from the tank to the pump with a cold engine will check the check valves. Do the same with a fully warmed up engine will check the "vapor" pressure issue. Be safe please. Block the line from the tank(gravity still works). No fires please.

Steve H