Very nice narratives - Rhodes and Admiral. Just had this issue on 83 300SD - 3 nondriver doors lock/unlock slowly, then not at all. Your hints saved me time - cycle the driver lock and look/feel/listen for which ones TRY to work. On mine, the Front pass lock was totally dead - only one door panel to pull.

Found a brake bleeder/vacuum pump kit at Harbor Freight Tools for $18. Seems to work ok....at least for awhile...LOL

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1983 300SD 307k miles
1993 Ford Escort Wagon 215k miles
1995 Isuzu P'up 214k miles