Got a trucker's parts house near you? That plastic hose matches a lot of the air lines for the braking systems on trucks. You may have to bring a piece an and let them match it, as they have it on rolls. If you can't find a truck parts place near you, truck stops with lube shops also carry it by the roll.

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002