Are you able to apply a reasonable force to the brake lever, or does it just go to the handle bars?
If you are able to apply a reasonable force but the shoes just won't grab then I would suspect that the shoes have been glazed over. Could have been from being overheated some time ago, or getting oil impregnated, or it is possible that they just got hard from age.
You can try taking a file/rasp and roughing up the surface of the shoes. If it is just a surface glaze then breaking through it will give you a better friction surface.
Rod