Hy hotrod dad, if this is a dual braking system then there must be a valve which separates the two systems, this valve will have a warning light connection to signal the driver if one or the other system should fail. This valve will either shut off or severely curtail flow to the failed system, that is what I think has happened in your situation. Whether or not that valve has or needs a proportioning function, it still must have the signal feature built into it. If you open a front bleeder and clamp the rear flex line and apply pressure to the brake pedal you might cause the valve to shuttle to the neutral position. If you can make that happen, then continue your bleeding while both systems are open. Disc brakes will gravity bleed if the master cylinder is mounted above the level of the calipers (if you are using a master cylinder for disc brakes), hope that helps.
