The (down)shifting hard on braking may well be caused by insufficient vacuum to the modulator as it shifts down. This could be your vacuum control valve being worn out or misadjusted or any number of vacuum leaks resulting generally in low vacuum. I finally figured this out a few months ago and got mine to downshift properly. Then a few weeks later it was doing it AGAIN- but at the same time the brake boost became erratic and the vacuum shutoff became slower. I replaced the brake booster with one from my parts car- no difference. After scratching my head for a while and just listening around the booster as it was shutting down, I bought a rebuilt brake booster. That solved all the vacuum problems and now the car shifts, brakes and shuts down properly

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