Since you have an 85 model you have a different situation than all other 617 turbo diesels. This should be the best shifting of the bunch. Unfortunately it is the most complicated too.
Modulators are fairly cheap and almost never the problem. The proportioning valve on the injection pump is over $100 and more often a problem. The vacuum amplifier is also never the problem, but is the difference between the 85 and earlier years. The early adjustments were so fickle that they added the amplifier to stabilize the controls and take boost into account.
Just about any symptom you can think of can be because of one area or another of this complicated vacuum system, but if you have over 180k and there is any doubt a rebuild is in order. The internal and external seals are due replacing if its a virgin.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician