Which primer handle do you have on your lift pump? Is it the white handle that you unscrew, or is it the later one-piece black one? If the earlier white one, it is very well known for leaking air, especially if it spills fuel when you manually pump it, and should be replaced as your first step.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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