My car has seemingly had this same issue. For my car, it was the rubber hose that fed the rear axle that had collapsed internally and needed replaced.
Rick
When I bought my 69 cuda I replaced the master cylinder, wheel cylinders (swapped the 7 1/4 rear for a 8 3/4). I bleed the front just fine, but could not get the back bleed. I tried the hand pumps, someone pumping the pedal etc. It ended up being the hose that connects to the rear.
