What is this vac. pump everyone is talking about. My 1987 300DT has 151,400 miles on it.

Unlike a gasoline engine, diesels do not create vacuum in the intake manifold so they have a mechanical vacuum pump to support the power brakes and other components. It looks like this, and is bolted to front of your engine:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ426P1BAXNL&year=1987&make=MB&model=300-DT-002&category=All&part=Diesel+Vacuum+Pump

The problem is, if they fail they can cause significant engine damage by getting small parts into the timing chain area. The design of the pump on your engine has be upgraded over the years, it's not a bad idea to make sure you have the latest version installed. If you search on vacuum pump here, you will find a few ugly stories.