Well, I just finished replacing 5 of the 6 fuel lines to and from the IP on my e300. So far so good, no starting or running issues, but what a messy job. Yes, the SO oring and the Banjo bolt line are next when I get the IM off for EGR cleaning.
I did come across this short thread and thought this 'could be' a reasonable theory concerning the chronic problem on the 606s and other models. Curious to know others thoughts as to its merits or not. I certainly dont have any expertise here, but it does somewhat remind me of the cigar hose purpose on the 617(?).
Apparent Air in fuel system, Mercedes-Benz OM.605/6 W210/W124 and 'G' Wagen
TLDR: Lift pump de-pressurizes the fuel line below the vapor pressure of diesel, ergo bubbles.