There's a whole thread of this somewhere in the past. I believe it is quite simple. There are two heater hoses coming in and two heater hoses going out of the servo. Disconnect these lines from the servo and hook them together with two pieces of pipe (copper or pvc). Then the coolant has no hindrance to running freely thru the heater core.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13