Regarding the vacuum in both lines. If I understand your post, your saying regardless of coolant temp, pull both line (black and yellow) at the Temperature switch (in effect bypassing the 40C opening), place a T between the lines, install a vac gauge and test (engine operating). I also suspect that if one doesn't hear the typical vacuum leak down when a system is pulled apart then there is more than likely a leak. Then the search is from there (backwards and forwards for the vacuum leak.
About how many bar/inches of Hg would you expect to see with a properly functioning system? The same as measured at the engine vacuum connection?
Also, Is there enough similarity between the 116 and 107 ACC that a 116 ACC MB Service manual (specifically the vacuum diagrams) would work?
S, J.R. Brown
2000 G500 LWB Obsedian Black
2005 Toyota Tacoma Access Cab Off Road Sport
1993 Volvo 240 Sedan Anthracite
1980 450SEL Champange (owned it for 15 years. Great car)
1986 280GE LWB Anthracite (Sold it and kinda wish I hadn't)