what possibly could be stoping me from being able to bleed my rear brake line?

If the rubber brake lines are old, they may break down inside and get clogged, like a heart attack. Disconnect the union before the line and see if fluid can be pumped out, if not, then check further upstream. If all else fails, have your system pressure bled at shop.

I would pressure bleed.

your Brake caliper is messed up get a new brake caliper !

or your brake power booster and blow air through the line and see if thats clog there