Trying to fix a vacuum leak. For this and other reasons I have taken off much of the interior plastics below the dash, so: While it's in this state what is the sensor between the black and white check valve and the firewall, and how do I tell it's working? It goes thru the middle of the dash for some HVAC pupose.

The check valve seems to work fine; cleaned with WD-40 as mentioned here. Getting less out of the panel vents under load (A/C or no A/C).

945T and (unless I totally miss my guess) with MCC.

http://www.brickboard.com/FAQ/700-900/HeatingAirConditioning.htm#960ACVentStopsWorkingUponAcceleration

http://www.brickboard.com/FAQ/700-900/HeatingVacuumServoReplacement.htm

On my 93 there are two check valves, one goes to the temperature sensor in the dome light and the other applies vacuum to all control valves.
If you loose air flow during load one double action control valve is probably leaking from a cracked rubber bladder.
Check the FAQ.

Hth,
mila