What I don’t see in your picture is how the return line from the IP is routed to the filter block. There’s a special banjo bolt just ahead of the throttle bell crank of the IP. The hollow bolt had a spring loaded ball as a pressure relief valve. Pinching the return line should have tested this but maybe something’s not hooked up properly. Or the 606 has a fuel cutting device I don’t see. So...

This is easy for me to suggest from as far away as I am and all I have invested in your project but the next thing I’d do is bypass the big filter; that is, route fuel directly from the lift pump to the IP. Run a hose from the top fitting of the lift pump to the forward most fuel fitting of the IP.

Sixto
98 E320s sedan and wagon