Next time they're dragging, loosen the steel lines at the master cylinder. Lets hope they don't release because that would be due to brake fluid contamination, and that's an expensive repair. Next, open a bleeder screw on the caliper. If that lets that brake release, suspect a constricted rubber flex hose. The additional clue is the brake pedal will be higher and harder than normal. There's a bracket in the middle of the hose where rust can build up inside and squeeze the hose so the fluid can't easily release.