The most common culprit of locked-up brakes is a malfunction in the master cylinder. The master cylinder is the primary operating system that produces mechanical force from the brake pedal to pistons that operate the drum brakes. The master cylinder system relies on seals leading to the brakes to maintain pressure so the brake system slides to a stop with ease.
When the seals in the master cylinder break down, the pistons in the system do not return properly, and the line pressure to the brake pads becomes uneven. This chain of events causes the brakes to overheat and lock up. If just one set of brakes locks up, the cause may lie within a caliper piston that is dragging, ultimately causing friction and overheating the brake fluid that then throws off the in-line pressure of that particular brake system.
While replacing the master cylinder is the most expensive repair to fix the brakes, vehicle owners can have a certified mechanic check to see if replacing individual calipers and rotors might provide a solution to brakes locking up before embarking on more major repairs. Routine brake maintenance and brake pad replacement also serves as a preventative measure to reduce the risk of locked-up brakes.