The braking system is two parts. Mine used to do this and I found out I just had to tug a bit harder on the lever/pedal itself and it would go off. If this isn't the case, I'd pull off your rear rotors and check the pads on the inside along with the inside of the rotor itself and look for grooves where the pads may have been rubbing a lot. Parking brake system only utilizes the rear rotors. The fronts are only used for normal braking.