Its my limited experience with the bosch rotary pumps like the VWs used is that they have 2 pumps, a vein like pump to move fuel into it, then the piston style with wobble plate to get the higher psi for the injectors.
In many cases when they are parked the vein pump gets are crappy and clogged up from the wax/gel of diesel fuel making it impossible to move fuel from the tank. Just install inline a fuel transfer pump and it will run again for 30 bucks, a piece of wire and some time.
