Can't bleed air out of rear brakes

Try using shorter strokes (travel) of the pedal.
Sometimes with long strokes the wheel cylinder will suck air in around the piston cups.
The power bleeders are a great solution because they force fluid the opposite direction causing those cup seals to flare/seal and push the air bubbles up and out to the master cylinder (the direction the air wants to go anyway) :-)

The Fords that have the slave built into the throwout bearing are a real SOB to bleed without a power bleeder, because they have a larger hose and the air tends to want to go up the hose instead of down and out the bleeder valve.