Weak vacuum signal to W124 300TD climate control

[240dddd]

I tested the climate control vacuum pods for my 1987 300TD this last week. (PO claimed he had replaced them, but the climate control seemed not to function as I expected). The pods actually checked out ok, however I noticed that vacuum coming to the switch-over valve unit seemed low as the vacuum gauge fluttered up and down around the 10 inch mark.

I did a quick test coming off the vacuum pump and the gauge held steady at the ~22 inches.

I then blocked off the two lines running to the EGR system (red X's in picture) and re-tested vacuum at the switch-over valve under the dash. The gauge went up a bit but still fluctuated a lot.

I then blocked off the line running to the vacuum control unit (purple x in picture). Now the vacuum was ~20 inches at the switch over valve.

Since I plan to delete the EGR, it seems that weak vacuum would be caused by the vacuum control unit or something else connected to the VCU.

Questions:

1. Is the expectation that vacuum measured at the Switch-over valve would be close to 20 inches? Am I interpreting correctly that 10 inches or less is a weak vacuum signal?

2. Is there common culprit around the VCU I should be looking at?