[wslade2]

I had the same issue but it was the fuel valve not providing a positive close off. Ultimately discovered deteriorated (as in gone) o-ring in the fuel valve.

I think the backflow not working is in the setting absolutely full tanks and dihedral causing the return port to be submerged. What is the fuel status in tank? If low and still leaking my guess would be the valve. So a test might be getting that fuel level down some below the return port and seeing if still leaking. I currently have my upper wing panel off fixing vent issue and installing CiES probes. I'm pretty sure return port is at top of tank.

However I am an E model and somewhat different from you, a G model with a unified system, with my valve that has to select between aux and main tanks. On the surface would anticipate return line in similar top position and valve construction similar but simplified for your model.

My check valve is located near the forward wing root under an access plate also with multiple electrical connectors.

I posted in another thread rebuild of fuel valve as no one rebuilds these for us.