For all the work you're doing on that AC system, you need a service manual. If you don't have one, they're available all over the internet.. Google is your friend..
I don't have a GMC service manual within reach but I do have '71 C/10 manual.. For what you're doing, a Chevrolet manual will suffice.. In section 1-A page 31, 32, and 33 are the cable and vacuum line diagrams.. Shows where each vacuum hose connects to those "flappers", as you call them...