If the bores of the wheel cylinders are good and not pitted badly they can be saved. Do like hoggyrubber said mount them in a vise and drive an allan wrench in the borer of the broken bleeder. This wrench must fit really tight if you cant find a tight fit measure the flats of the next size bigger allan wrench and drill the bleeder this size and broach the wrench into the hole. When this is done heat the casting around the broken bleeder really hot with a torch and remove the offending bleeder with the allan wrench you drove into it. I've used this method miny times without fail with no damage to the cylinder. A allen wrench makes a really good easy out as they will not break off in the hole and tend to be a little springy.
Also the cylinders can be taken apart at this time with a greese gun with out damaging the spring or pistons. The old hammer and punch method will generaly ruin both.
