If the 621 is like the 616 and 617 all of the lines after the lift pump should be pressurized. I would try changing all of the fuel lines. They could probably use it anyway. Those old braided hoses are almost impossible to find pinholes in.

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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