Do you have a standard banjo bolt on the return line AT THE IP, or do you have the special orifice "overflow" valve installed? If you have a standard banjo fitting, that's your issue, you'll never build any pressure in the IP's fuel rack. The correct orifice valve looks like a banjo bolt, but has a very small drilled orifice on the side instead of the normal hole. It also looks like it has a ball bearing pressed into its head.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]