Is there a 'tree' on the firewall, sort of near the power brake booster? Most all FoMoCo vehicles had a plastic tree that had multiple ports for all vacuum operated things. The large hose that runs from the intake to the booster in your picture generally would connect to the tree rather than directly to the booster. The tree would have a connection to the vacuum reservoir, a largish plastic chamber stuck wherever they could find it, which in turn was 'check valved' such that when manifold vacuum dropped due to lead footing the throttle, there would still be vacuum to operate the heater mode flap, the power brakes, etc. My '85 that has a vacuum operated EGR. Period. Has a vacuum reservoir hidden under the air cleaner box.
If you poke around under the hood you should be able to find a vacuum line diagram showing all things vacuum, and how they are connected. I found a GIF file, but don't know how or if it can be stuffed in here... It DOES show a multi-ported connector that is between the intake manifold and the brake booster.
I think someone has 'adjusted' the vacuum lines, and disconnected the heater control vacuum.
tom