No luck so far.

I replaced the MC with a remanufactured one (Cardone) from a local part store. Simple job, really. Just one of the brake line fittings hard to get to. Cost was about $50 after the core. Bled all four corners again (I am on my second quart of fluid now(!), but - still have the sinking brake pedal.

I went over all the brake lines I can see, and there is no visible leak, and no loss of fluid. Brakes work fine, but the pedal action is hardly 100%, and I would hate to make a panic stop this way from hwy speed.

At idle, the rpm will dip slightly when the pedal is pressed, then rise to about 1K for about a second when the pedal is released. I disconnected the vacuum line at the booster, and there was plenty of suction. Power assist is good. The car does not have to be driven until I get everything working right, so I will keep at it when I have time. If necessary I will replace the MC again. I'll post a solution if I find one.