I am not exactly sure if this is what you are looking for. But, I am going through my AC Vac system on my '85 190d and I found this to be helpful in diagnosing one of my problems. You will have to search for it after you figure it out as he doesn't tell you how to fix it.

http://www.cacavas.com/mb/TTM.html

Read it all. But, the "CHECKING FOR VACUUM LEAKS" Section is where I found what I needed but, it is for the switch over valve.. Hope this helps..

Also, If you do remove the switch over unit, Be careful not to break the cliips holding it on. I found it helpful to remove the kickpanel below the glove box to get to the right side clips. Also loosen or cut the nylon tie holding the bundle of wires closest to the door after you remove the kickpanel. It makes it much easier to get the switchover valve into a posistion that you can test it...

Good Luck...

EDIT.. HMMM this is my 1st post here. Maybe I should look at the dates before posting again Huh??