The quick-and-dirty way is to clamp the hose and disconnect the idle coolant-circulation pump as shown in the picture.
OR, the more detailed way is to start at the yellow vacuum check valve near the fuse box. There are two circuits to test. Disconnect both the green hose with red stripe and the red hose with gray stripe (on my '89 190e 2.6, anyway.) Use a hand-held vacuum pump to test each circuit's ability to hold vacuum. In my case, the red one that goes to a vacuum storage element was bad, so I just plugged that lead to the check valve so the rest of the system would work, as in the next picture.
If it is the green line that won't hold vacuum, there is a lot to check out and you would want the manual on CD, or to take it to a mercedes tech. But first, sometimes, the gaskets in the monvalve itself go bad -- around a $30 part and a simple DIY project. Just follow the threads on the forum on how to get to the monovalve and check that it holds vacuum--if not, that might be where your leak is. Hope this additional information helps, as it may not be sufficiently similar to your model year. Proof that a hand-held vacuum tester is a must for the Mercedes DIYer! Cheers, Andrew Seidel.