It is routed to the vacuum modulator located on the side of the transmission.

Looks like:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=24L15MYL825F1F6K9T&year=1984&make=MB&model=300-SD-002&category=J&part=AT+Modulator+Valve