Rear brake is mechanical and hydraulic. You have to turn the outer adjuster screw to where the petal lifts up from the floor board about 1/2" from the bottom of the lever. Problem is that many snap from corrosion over the years. You can take a chance on it. If you get it adjusted,then you can resume bleeding. Item #34.
If I remember correctly,this screw is reverse threads.Turn left to tighten,right to loosen. If that doesn't help you may have a frozen piston or seal are bad. Shows the piston and seal kit if you haven't already replace them.