OK, Isolated the master - Good
Blocked rear output at the combo valve - 1\2 pedal
Rear hard lines, flex line and y-block - clean and clear

I pulled the combo valve, disassembled it as much as possible, cleaned and soaked with penetrating oil. I then used a small screwdriver to move the shuttle valve, but was unable to move it back non-destructively (even with compressed air). It looked like it was stuck open, so there would be flow to the rear. I thoroughly cleaned and reassembled the valve, installed it and bled the brakes. No more air, the pedal is rock hard, but only goes down half way. I'm thinking it is hydraulic lock in the combo valve. The slider must be disadvantageously stuck and when the valve opens the rear channel, no flow.

Any other ideas or avenues to explore? I'm almost ready to get one of those Wilwood adjustable combo valves and plumb it in...