[ssjones]

Eureka! You nailed it. The module beside the brake booster has a hole in the middle, the fitting must be missing, hopefully not broken off. This would also explain why the vent function for HVAC won't work! (inoperable for over a year!) The HVAC control has been stuck on window defrost, I thought it was a problem with the dash control unit or the vacuum line connections under the glove-box. Another potential problem, the bottom of that housing is broken and I can see a little exposed foam.

I'll snap a photo and post this evening. Any way you can take a photo of the fitting to your line?

Thanks for sticking with me on this, the HVAC problem has mystified me for over a year!