[kda406]

For posterity, it was the Master Cylinder.

First I verified the MC was not plumbed backwards. I finally broke down and bought a new (instead of another remanufactured) MC. I bench bled the MC and mounted it. I used the screw in tool in the proportioning valve and a power bleeder to get all the air out. I bled them forever, until all the fluid was clear (new).

I really didn't think that would solve anything, but was shocked when I drove her and I had normal brakes. No build-up delay, proper toe-play, and the rear brakes no longer lock up at really low speeds. I don't know what was wrong with that MC, but I'm glad to have the confounding issue behind me.

Thanks for the help, guys!

-Kyle