Yes, a thump downshifting is an indication of a vacuum problem. Push the far right button on the heating panel. This shuts down the heater and excludes any vacuum problems in the heating control system. Then plug the line going to the doors (yellow line just to the right of the brake booster I think). Then go for a drive. If it shifts better, you know the vacuum problem causing the harsh downshift is either in the heater or the door locks. Then turn on the heater. If the downshift is still smooth, you've traced the problem to the door system. If it is harsh again, it is either just in the heating system or in the doors and heating system.
If it is still harsh with the doors and heating system excluded the problem is in the brake booster, shut off valve system or transmission control system, or vacuum pump itself.

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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