So it’s not the overflow that has a problem. If i hook the lift pump output fuel line straight to the injection pump return, it will prime the pump perfectly, buzz, flush air, run and drive.

The problem is when i hook up the mail fuel filter, the pressure from the primer is lost to the return line. The fuel will just cycle and never enter the injection pump, following the path of least resistance.

There must be something in the return system that I’m missing or isn’t hooked up correctly, this mystery mechanism must create a greater pressure than the overflow in order for the fuel to cycle through the IP, and not just flow right back into the tank via return line. Does that make sense?
Ask away. I’ll go get a picture