Sounds like a fuel supply problem. Does fuel flow from the supply line if you remove it from the prefilter (inline filter)? Does the hand pump feel like you are pushing fuel when you pump it? Does fuel (and/or bubbles) move up the semi-tranparent fuel return line (from the motor side of the pump to the fuel filter)while pumping?
If these last events don't happen and there is fuel at the supply pump, I would expect the replacable check valves inside the feed (supply) pump
Bosch Master, ASE Master, L1
33 years MB technician