The problem is going to be the brake hose having collapsed internally. When this happens it allows brake fluid thru to the caliper when you press the brake pedal, but will not allow the brake fluid to travel back up towards the master cylinder when you release the brake pedal, thus causing the caliper to remain engaged. The usual cause of an internally collapsed hose is due to brake fluid boiling inside it or contamination of fluid. Usually when the caliper sticks the brake fluid boils inside it and also in the hose due to the overheating condition. This is why I always replace both whenever one of the calipers has been sticking.
Replace the flexible brake hose at that caliper and you will be good to go.