I have a british sports car and the front left brake disc brake would lock up on occasion. The caliper was removed and rebuilt, new pads installed, flushed out and renewed the brake fluid, checked the pdwa (a switch which detests loss of brake pressure between the front and rear). The only thing left was the flexible brake line to the caliper. Although it looked perfect from the outside, it had deteriorated internally creating a one way effect not allowing fluid to return when releasing the brakes.