I have the repair manual for my '75, and not all vacuum fittings have a hose coming from them. However, It took me quite a while to figure out the diagram and realize what all the notations meant. For example OA/RA means "outside air"/"recirculated air".
On my '75 there is also a switch built into the temperature slider, and it serves as the Max A/C control. If the slider is all the way left (position 1) and A/C is on, then you get high blower and recirculated air (max A/C). If the temp slider is one click away from far left (position 2), the blower operates normally in A/C position and you get outside air.
My manual has a table that shows each vacuum door and whether it has vacuum when the A/C controls are in each position. I used that table and a vacuum gauge to verify that each door was getting vacuum when needed. Then I had to lubricate the linkage for each door and make sure all the retaining clips were on correctly. I also had to repair some of the vacuum hard lines, using small sections of regular vacuum line as a splice.
Bruce