When it's sticking, open the bleeder screw to see if it releases. I'm betting it will. That proves the brake fluid is being trapped and there's nothing wrong with the calipers. The most common cause is a restricted rubber flex hose. Get the brake to lock up again, then loosen the steel lines at the master cylinder. If the brake does not release there, replace the flex hose. If it does release, we have to consider the brake fluid is contaminated with a petroleum product. That will be a very expensive repair. The clue is the rubber bladder seal under the reservoir cap will be blown up and mushy.