Rear calipers have two bleeders each and both must be bled. Fix the front before you screw around with back, so you don't fight two problems. If you are bleeding it correctly check rubber hoses.
Are you sure your problem is with rears? If you feel it must be the rears because the brake peddle is soft then goes hard, does that mean that if problem is with front then the peddel is hard then goes soft?