Allright, so here is the latest.

1st, I have NOT yet isolated the front brake channel and tested.

What I have done is re-bleed the brakes (using a 2.4 under the pedal) after re-routing both MC-prop valve lines such that all of the coils are below the level of the MC mounting. I also visually verified brake operation at all four corners.

The result is a HARD pedal that feels as it should, and normal braking is as it should be. That said, no matter how hard I stand on it at any speed, not even the rears lock up and it gently slows to a stop after a much longer distance than it should. The pedal does not pump up, and braking begins immediately upon pushing the pedal.

Could this symptom be caused by an MC mismatch? I replaced the stock replacement (Big A Auto no less!) MC with a KH unit from Rock Auto listed for an 89 Diplomat. It is listed as having the desired bore. Several aftermarket brake manufacturers list "Master cylinder bore size too small for the system volume requirements." in their troubleshooting guide for "Poor Brake Pedal" but the symptom is no more specific than that.