The O-ring seals on the delivery valves on top of the injection pump are a common leakage point on the 603. As said above, unless you're just pouring diesel out of there, they shouldn't cause the starting issues you're having.

Your return line should be 100% free of bubbles and foam. Usually air enters on the suction side of the lift pump, take a look at that fuel preheater thermostat and the rubber fuel lines.

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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!)