Park on a slight incline, shift to neutral, put a block about 6" from a tire so you don't have to run after the vehicle when the brakes release, then loosen the steel lines at the master cylinder. If the brakes release from there, something is holding the brake pedal down, such as the brake light switch, or the brake fluid is contaminated with a petroleum product.

Contaminated brake fluid is a very expensive repair so lets hope the brakes don't release at the master cylinder. Now feel if one front brake is real hot. Loosen the bleeder screw on that caliper. If it releases, suspect a constricted rubber flex hose.