Dieseldon:
Your leak is in the actuator for the footwell heat. It's the only actuator that only works when the heat is on (I believe). Remove the panel under the driver's dash and you will see the actuator just to the right of the accelerator. It's not the one on the side of the heating ducts (that one does the defrost I think) but behind the ducts just to the right of the pedal. Very easy to test with a Mighty vac.

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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