Perhaps someone has bypassed the servo in the past. There shouldn't be a heater valve and a servo. The servo serves the function of a heater valve.
I don't recall anyone successfully rebuilding a servo on this board. A used, rebuilt, and the new solid state ones are your best choices I think. Or, bypass the hoses (which may already have been done).

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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