GOVERT

You are correct. It is the switchover valve. I ended up swapping out the one that I had just put on (used but tested out fine) with another and it did the trick. It needs some fine tuning, but it at least will shift now! Also, putting a dashpot inline with the VCV seemed to tighten up the shifting just a bit...

(I had all that is below typed out then did the switch, so I will leave it in the post for anyone who may have the issue in the future.)

:::::::::::::::::::::::::::::::::::::::::::::::

I was able to get a VCV that I KNOW 100% is fully functional. Here is what happens...

My reading taken between the switchover valve and the modulator drops to 0"hg as soon as the switchover valve nob gets even the slightest bit depressed by the throttle linkage. It is as though my VCV is not even there, however I know that the VCV will hold vacuum it just doesn't get to it - however the lines and linkage is all good, holds vac just fine.

Even disconnected all the lock/ACC controls from the main brake booster line just in case, still the same.