Welp, it was the same spot it broke before in the engine bay.
I had broken this several months ago pulling out the transmission dipstick. The rigid hardline coming out of the firewall snapped where the rubber line plugged into it. I snipped off the broken end and re-spliced.
Well, the line broke again, twice.
I found it by unplugging the line from the vacuum tree and blowing in it. Or having my son blow in it. I could hear it leaking, and traced it to my splice. The hardline had partially broken right at the splice. Snipped it and re-spliced, then found another crack about an inch further down the line. Snipped again and now everything works fine. My hardline coming out of the firewall is getting kinda short though...hope it doesn't break too many more times!
The partial break explains why sometimes after running a while or gunning the engine the AC would slowly switch back over to the front registers - finally pulled enough vacuum to do the job.
So, AC works again!
