I bought a metal bodied servo recently from Randy at European Auto Parts (?). Do a search on servo and his web address will come up. Very good to work with.
Seems much better than the original. But you won't have one on an 84.
For anyone looking for a servo, they were also used on Chrysler Imperials (?) so you might find a used one there also.

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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