That's not a rare pump. It is standard on early 617's. It can be rebuilt without removing the whole pump from the engine. You don't even have to remove the radiator, just the shroud. Remove the outer cover, replace the diaphragm and check valves, reassemble. Should take less than 2 hrs.

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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