It really shouldn't be that complex, I pull a vacuum on the return outlet of the pump till fuel comes out, if the system is totally dry I might open the injector fittings at the injector while someone
cranks till fuel comes out, then I crank till it starts which should be nearly right away,
inside a minute it's fine. If this isn't happening, you have a leak somewhere or some other related problem. The system is self bleeding
essentially, until it's sucking in air somewhere. Also, that seeming to pull fuel into injectors you are seeing with the clear lines is quite likely an optical illusion,
it's not possible for the injectors to suck fuel back in. There is also very, very little flow/pressure in those overflow lines, it's why that little rubber cap on the end
one is sufficient. They are there to provide a path back to the tank for any fuel that leaks around the nozzle inside the injector body, fresher the injector, the less fuel in those lines ime.
Put a long hose on one and run it and see how much fuel comes out, it's generally not a lot. Fuel or air in those overflow lines
should have zero impact on how the thing runs, other than making a mess they will run just fine with no hoses on them at all.
Beyond getting the IP full of fuel, and doing a gross bleed of the injector hardlines, that's it. The system bleeds itself while running.
You might start with soaking the whole area in brake cleaner to remove any fuel, blow or let it dry off, then run it and look for weeping.
You can also put a long rod or screwdriver or mechanics stethoscope on each injector and pretty clearly hear when they pop at idle, if there is
air which compresses, they will not pop, fuel does not compress and makes them pop(which is to say, open or spray).
When was the last time that pump was re-sealed or the injectors rebuilt?