Thanks for your interest. I agree there must still be air. I've tried gravity bleed, vacuum bleed, pressure bleed and the two person push the pedal as the bleeder is cracked open. I haven't tried your method, but will have to give it a try once the temperature gets warmer. I have gotten a solid pedal, but not great feeling brakes. The top 1-1/2" does nothing, then the vacuum boost is felt helping and the brakes start engaging. By the time the pedal is solid and about 2" off the floor, it doesn't have much power assist and you just have to push hard to stop.
FYI for those gravity bleeding and getting poor or no flow from the rear. I had to break the fittings at the proportioning valve (PV) to get flow again to the rear brakes. I'm not sure if it had created an air lock, but I had hardly any gravity flow to the rear until I bled the PV fittings. I tried gravity bleeding with the PV fully open and 50% closed. Once I had flow again, the position of the PV didn't seem to affect the gravity flow.
1966 Mustang 2+2, A code, AOD AT, PS, PDB, AC, Pony deluxe interior