Those had a adjusting pawl like on old Clarks with inboard brakes. It's a little arm that fits through a hole in the shoe and holds it from retracting. As the lining wears and the shoe moves outward the pawl is pushed with it and stays there. There should be 2 nuts on the backside of the backing plate, at about 2 o'clock and 10 oclock that you can try to loosen, or maybe turn inward that would make the shoes release.