That's good news. Maybe starting a separate thread on 80 Vacuum controlled transmission troubleshooting might be good since there isn't much on here. I would do two things. 1--monitor existing vacuum in the configuration where it wasn't shifting. 2--monitor vacuum in your new experimental configuration. 3--disconnect all vacuum connctions to the transmission except for the Mity Vac and feed the mity vac into the passenger compartment and drive around pumping different amounts of vacuum into the mity vac to see how the shift patterns change.
What would be very useful on the forum would be a conceptual analysis of how the transmission is controlled strictly by vacuum? On bowden/vacuum systems, vacuum controls harshness of shift and bowden controls shift points. Vacuum must do both on your model. Given the fact that you're now flaring, I'm assuming that your experiment increased vacuum to the transmission. You could simply remove the vacuum line fromthe transmission so it is getting no vacuum and see what happens. Given what has already happened, I'm guessing it wont upshift at all.
Another possibility is that the modulator is failing but I would think it is far more likely that the problem is in the amount of vacuum--which could also be a failing VCV if you have one.

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