OK, just got off the phone with a Wilwood tech. He said the 15/16" master might have been marginal, but the current 1" bore should be plenty. He's sure there's still air in the system and recommended the 2-person bleed method so I'll have to wait 'til the weekend for my son to return from vacation before trying that.
He was as puzzled as I about the fluid back-flowing up out of the reservoir's base port. His thought was the piston seal is defective, causing the fluid to push past it and bypass back into the chamber. Which would explain the lack of rear braking.
Any volunteers with an older setup (pre step-bore) willing to pull their reservoir cover and observe what their fluid does when the pedal is (lightly!) pushed?