[Schurkey]

Originally Posted by Kenth

all mechanical fuel pumps have a check valve that prevents fuel from "flowing back into the tank".

Some pumps have two check valves--one inlet, one outlet.

Some pumps have six check valves--multiple inlet, multiple outlet for higher flow volume. No doubt over the course of automotive production, there's pumps with other numbers of check-valves.

It's entirely possible for them to not seal completely. Even slight leakage of an inlet and outlet will drain everything from the needle valve to the level of the fuel in the tank, given enough time.

Thus my suggestion to connect a vacuum pump to the inlet of the fuel pump, to see if--or how badly--the inlet check valves leak.