Air always coming out of center vents - not holding vacuum in "0" position. w201

[Chowbow]

Hello climate control experts. This is my first foray into fixing my climate control problems. Here is how I have tested the vacuum on my 1992 190E 2.6:

I took the green and red striped vacuum line by the fuse box and pulled a vacuum on that line with my Mityvac, while going through each vent position on the push button unit. All of the vent positions hold vacuum except when the pushbutton unit is set to the "0" / off position. It doesn't hold any vacuum nearly like the line is disconnected.

Another symptom is that when the system is in the "0" position, the blower fan is off like it should be, but air comes gushing through the vents when the car is at speed.

What pod / vent controls this? Is it the main air flap or recirc pod? I am not sure where to start, and I'm a bit new to all of this. I also have the clicking noise behind my glovebox and heat out of the side vents ONLY when the car is at load. I have checked the other engine related vacuum lines and there are no leaks, I have replaced most of the vac lines and rubber in the engine bay.

Any ideas? TIA.