Here are some vacuum diagrams to help you diagnose your W123, W124 or W126 diesels. The 240D diagrams were looked up for a 83 year model, but should be pretty much the same for 82-85. I've also added some wiring and flow diagrams. Also note that there are year to year (and sometimes, throughout the year) changes and these diagrams may not reflect everything exactly as on your model.
'86-'87 300 SDL, '87 300D vacuum diagram
81-84 300D 49 STATE TRANSMISSION VACUUM
http://www.detali.ru/cat/oem_mb2.asp...SGR=195&SGN=09
Cruise Control Repair Articles
http://www.peachparts.com/shopforum/...d.php?p=883130
Edit:http://www.peterschmidtransmission.com/vacuum.htm also has a good deal of vacuum diagrams available.