I am putting the under-dash parts back together, so it is time to give up on finding a new vacuum diaphragm for the speedometer reducer. I patched the underside of the old diaphragm with PVC tape and hope it holds for a while. If this doesn't work, I will see if a speedometer repair shop can fix it. I am disappointed by GM engineering in this case, because it appears that spring pressure from the vacuum inlet caused the problem by pushing on the rubber diaphragm. This could have been easily solved in the original design.
I am also surprised that no one could tell me if a diaphragm that fits can be found or not. This speedometer reducer design seems to have been used by GMC and Chevy for many years starting in the 1950's and continuing into at least the later 1960's, so many trucks must have them, and replacing the diaphragm is easy once the reducer has been removed, which I also found easy to do after pulling back the instrument panel.
Since this all started with trying to replace heater/defroster ducts, I can also report that the basic ducting is relatively easy to work on once the instrument panel, heater/defrost controls, heater box, and glove box are moved out of the way. Because of openings in the cab (such as a deteriorated steering column boot or gasket) and with the old ducts either missing or falling apart, the entire heater box and other openings were filled with mouse nests (and mouse bait had been stored in the heater box), along with more nests in other hard to reach under-dash nooks and crannies. So this project was really needed. However, it was not easy to install the new ducting because of some odd shaped inlets and because the Gates brand 2 1/2" diameter duct tubing is a bit smaller than the inlets, which made it very hard to push on. I finally had to cut (but not remove) the embedded spiral plastic cording at the duct ends to stretch them over the inlets - then I applied a few layers of PVC tape and a zip-tie to prevent leaks and hold it all together. Now I am hoping not to have broken any of the many electrical connections while pushing things around and that it will all work without further repair.
