If my brakes locked up like that, I would disassemble the caliper and find out what happened. Changing the fluid won't fix that I'm pretty sure. I would guess some type of contamination has gotten in the system, debris or condensation, and caused the brakes to bind up. That happens to a lot of the old bikes I buy to sell and fix up. Usually, I don't even have to replace any parts. Just clean the system out and make sure everything is not pitted and is clean. Coat the parts with clean brake fluid and reassemble. Then bleed them. If the caliper looks ok, then I would do the same to the master cylinder.
'14 Ness XC