Wheel reinvention. The hard lines and all the fittings all the way back to the tank are 8mm. Replacing the rubber with 3/8 will do squat. Stick with 8mm or 5/16 and make life easier for yourself. The lift pump on the 603 is the same internally to the 606. 8mm is considered marginal by Bosch, but it works fine on these cars since the orifice in the return side of the IP is so small that they can never develop full flow.

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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!)