I thought I already posted this but I can not find it now. Does anyone know the function of the vacuum valve on this box? TIA

 

It controls the flapper door for air recirculation vs outside air. Vacuum to both ports gives you 100% outside air. vacuum to the upper port gives you 20% outside/80% recirc air. And yes, this is in the CSM, chapter 1C.

 

Thanks again Joe. Apparently it is a BOP thing and Chevrolet did not use it.

 

Interesting. I would have bet that the HVAC parts were the same on all A-body cars within a given year range.

 

I found it here

https://www.chevelles.com/threads/19...eeded.1142315/