Start buy looking at the vacuum diagrams so you understand what goes where:

http://www.peachparts.com/shopforum/showthread.php?t=192440

You will need to isolate sections of the system until you track down the leak(s). It does not make sense to replace all the hard lines, but the rubber connectors and fittings are cheap and available:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ429618D9FX&year=1981&make=MB&model=300-TDT-001&category=All&part=Vacuum+Hose

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ429618D9FX&year=1981&make=MB&model=300-TDT-001&category=All&part=Vacuum+Hose+Connector

It's not really difficult just tedious, you need to be systematic.