I just got finished with a 71 Nova disc brake conversion and had the same issue. I wound up having my buddy pump and hold the pedal and I bled the lines between the M/C and the prop valve, and the outlet fitting to the rear brakes. It took a lot of pumping to clear the air out of the rear lines. My Vacula bleeder was not effective, I think the prop valve limits the flow to the rear brakes quite a bit.