I replaced the ac/heater control switch with a newly purchased one on a 71 Cuda. I also rebuilt the box, cleaned all the lines, and function tested all the actuators/doors before installation using a Mity-Vac handheld unit. Everything is good there. I pulled the vacuum hose at the manifold and tried to pull a vacuum back to the switch and could not hold vacuum. I pulled the vac hose at the switch just passed the check valve and plugged the end, pulled vacuum again at the manifold end and held vacuum. I can pull the "cannon plug" and have "holding vacuum" on each port/line (I had performed this previously on the work bench to function test all box components/doors, etc.)

I found this issue chasing down an oscillating manifold vacuum reading when trying to tune my carb. Removed the AC/Heater manifold vacuum line, plugged off the manifold port and manifold vacuum improved. That's when I started chasing the vacuum leak down the ac/heater system.

So, my question is; based on what appears to be a vacuum leak within the switch body...this new switch is probably bad, right? Should it not hold vacuum internally? Anything else I might be overlooking here? Anybody got some trouble shooting techniques to share? It' been many months since I purchased the new switch, so I doubt I can get a swap out from vendor.....you know how it goes in that regard.

Your inputs are greatly appreciated.