Okay, through the process of elimination, I was able to find out exactly which vacuum line should be T'd into, on a 13B, for a vacuum gauge to give off a proper reading. I have provided a picture for easier reference. It is the solenoid/switch/whatever on the passenger side, against the firewall, closest to the fender. And, you must "T" into the top vacuum line. A better move is to get an "F" vacuum fitting for an easier fit. It also saves from you having to cut into your stock vacuum line. Handy if you ever need to remove the vacuum gauge's vacuum line.

The other two vacuum lines on those two canisters will give improper readings. Like I said, process of elimination.