If it is only a couple brakes consider...Have you looked at all the brake hardware? Are drums in need of turning? Is it good? Are pads on correctly? Brake pedal return spring good? Brake pedal pivot lubricated correctly? As 3b stated, linkage at the correct tightness to allow piston return?

If all the brakes are dragging, I would lean towards linkage, return spring or in need of lubrication.

Good luck!

Chris