The fuel can't be pushed any further than the fuel stop solenoid unless the ignition is on, and it won't go back to the tank via the return line because the pump isn't moving/spinning. There's a one way valve inside the primer head that is supposed to prevent fuel from running back to the tank, so assuming that all is well with that then I would not suspect that. There are so many places for air to enter the fuel system, it's unreal. We all hear about the leak off pipes and the primer pump because these are the most common places for air ingress, but you also have the banjo nuts on the fuel supply and return pipes on the injector pump, these have copper washers that have to be replaced each time they are disturbed, the top cover of the pump and the throttle arm both have rubber seals, as does the advance solenoid, and the boost compensator diaphragm, either of these could be letting in air. If the motor starts good and runs good then I wouldn't worry to much, but if you that you still have an air ingress problem that's causing prob's then you could put a piece of clear pipe on the return side of the pump and look for bubbles there.