The defrost on my 1985 D100 was stuck on, so I tore into the HVAC control system. There is a plastic connection piece about 6 inches behind the controls that for some reason allows you to disconnect all of the vacuum lines?...why it's there, I don't know. One side of the plastic connection was brittle and had broke into a hundred pieces, so I was getting no vacuum to the controls. I cut the snap connectors off and I bought some 7/64" ID hose and manually connected all of the lines (trial and error). There was one rogue, red vacuum line coming out of the controls that had no destination, so I had to plug it to maintain the vacuum within the system.... The truck actually idles better, as the loss of vacuum was not pulling the vacuum advance on the distributor.

If you need the vacuum line diagram, you can email me at bobschieffer@yahoo.com