Air can only enter the system where it is under vacuum, or before the lift pump. Any leaks past the lift pump will just leak fuel. Do you have the old (white) style primer pump? I would try replacing that first.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine