have you checked the tail lamp bulbs to see if they are good or if they are blown ?
its possible that this has been happening for a while and those bulbs have already blown out and the cab brake lamp will be next,,then you won't have any brake lights.
if this is indeed the case i would suspect that the problem is with the brake lamp switch it self,,the switch is either defective or it is jammed in the on position.
you should use a continuity meter to check the bulbs and a test light to check for voltage at the tail lamp socket.
check this stuff first,,let us know what you find.