The lines between the injectors aren't really important regarding air or vapor in the lines. They are overflow from the injector only. If your injectors are tight, it can take a while for all the air to bleed out since very little fuel flows there.

Where to watch for air is in the return line from the secondary filter back to the fuel tank. With a clear line attached in place of the rubber line, you should see a solid stream of fuel returning to the tank. Unless you live in Siberia, bypass that fuel heater, it is a source of constant trouble and usually the first thing to start leaking air in the fuel system.

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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]