I used silicone hose for that, from ~3 ft I bought on ebay. I recall 42 mm, but don't quote me. My nipples weren't very pitted, but silicone might seal better since more flexible. I got it because it should last much longer. I doubt there is more pressure with the heater on. In a 300D, (only W123 diesel I know of), there is just the mono-valve in the heater loop. But, even closed, the line pressure would be about the same at the bypass location. There is also the Aux Water Pump (unless you removed it like me), but doubt that builds much pressure.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans