Checking the vacuum on the climate control unit
 

Situation: 87 XJ-S V12 thought to have a Mk III climate control (simply because I don't see any of the pushrod arrangements on either side when I remove the foot vents and covers).

The system works, but poorly. I get very very little airflow out of the center vent or the side vents. Given that both blowers work, I would expect to feel lots of air coming out of them.

I've checked the check valve leading to the vacuum tank (I can blow/suck one direction, but not the other), but that doesn't tell me much about if there is a leak downline of the valve.

So, how to diagnose? How much vacuum does the system require to operate, and how long should the tank hold the vacuum? I supposes I could use compressed air to pressurize the system and check for leaks, but not being familiar with how the vacuum actuators work, I don't want to damage one by putting too much pressure on them.

Thanks,

John