I suspect RUST. If you have any oil and acetone or nail polish remover, mix up a small batch of a mix of the two, mostly acetone and only a bit of oil, and soak the bolt. Wait 24-48hrs. and use the lever trick making sure to add wedges or spacers so the brake can't close when you squeeze the lever. Then squeeze the lever like you're coming into an intersection at high speed, and see a cement truck coming from the right on a collision course.

BTW0 the worst case scenario is that the bolt and cable are rusted together. In that case, you can buy a new bolt (they're still available) and cable and start fresh. Simply cut the old cable above the bolt.