The brown tube might be for the engine stop switch. I believe the line to the ACC system is red with a green stripe. Follow it from the yellow check valve ahead of the brake master cylinder. On the cabin side, one line goes inside, the other line goes to a reservoir somewhere under the fuse box.
If that line holds a vacuum, check if the source provides a vacuum with the engine running. I don't know what it should be but 14" Hg is reasonable.
Then check if the distribution block on the right side of the center console is working. A functional diagram is available in the MB W126 CD manual but it's not necessary for finding faults since it's easy to follow vacuum lines to their actuators.
The easiest actuators to get to are the recirc actuators behind the glove box. Pry off the center posts of the 7 clips holding the glove box in place. Pry off the glove box light and thread it through the hole. Pull out the glove box and you should see a couple of pink actuators each with two vacuum ports. To the extent that you can get a MightyVac hose on the vacuum ports, see if they'll hold a vacuum. The actuators work in parallel so you can test the chambers in pairs if that's easier. If you reach to the left of the glove box near the top of the dash you'll feel the defrost vent actuator (or closer since default is open). It's the other dual chamber actuator in this system. Test that one as well.
That will tell you if those actuators are good or bad.