First of all, a Mityvac is indispensible if you own a Mercedes diesel. If you don't have one, get one.

The way to trace a vacuum leak is to follow the vacuum circuit back from the pump, and follow each branch. Every place the line branches, break it apart. Use the Mityvac first to make sure the branch is seeing vacuum from the pump. Then pull a vacuum on the line to see if it holds. If everything is ok, the problem is on the other side of the branch. If not, it's on the side you've opened. Reassemble, and keep going until you've located the problem.