Have you adjusted the rear brakes manually?
With the wheels and rear drums on, turn the brake adjuster (star wheel) so the bottom of the two shoes move apart. You do this with a flat blade screw driver through the little inspection hole at the bottom of the backing plate. If your rear brakes are set up properly, the star wheel only will spin one way, like a ratchet. The automatic adjuster prevents it from turning the other way.
With the wheel lifted off the ground, adjust the brakes until you feel the shoes drag a little on the drum when you spin the rear wheel.
Do this to both sides, then step on the brake pedal and try adjusting the rear brakes a bit more, if the shoes no longer drag.
If the pedal is still soft, bleed the brakes again.