Either you've got a bad overflow valve or your sucking air. Have you taken the bowl off the rock catcher? Have you verified that the o-ring is sealing and not allowing fuel to drain back.
You said you pressurised the tank to get a prime. Are you gauging this pressure? Beyond 5-6 psi, you run the risk of ballooning your tank. If you're pressurizing, cap the return line. This will cause fuel pressure to act on the suction side. Look for anything collecting fuel. If no fuel escapes, no leaks.
If you look at fuel lines to and from injection pump body, you will have a banjo line from the lift pump . That's inlet and will have a hollow screw. The return fitting will be your overflow. It will either be a banjo screw with a spring loaded plunger or it will be on a stand-off valve with a hollow screw. In that case the stand-off will be the overflow.