I would continue to use the rust remover/penetrating oil products, put a pair of vice grips on the chuck grip and use a hammer to tap the grips. Hopefully this will free it up. It's not very elegant and you probably have already tried similar methods but it's the best idea that I have. Also you could soak the chuck portion in penetrating oil (chuck submerged only) overnight. Good luck!
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I would continue to use the rust remover/penetrating oil products, put a pair of vice grips on the chuck grip and use a hammer to tap the grips. Hopefully this will free it up. It's not very elegant and you probably have already tried similar methods but it's the best idea that I have. Also you could soak the chuck portion in penetrating oil (chuck submerged only) overnight. Good luck!