An air brake system is equipped with a service brakes and a parking brake. A control valve for the parking brake vents air therefrom to apply the parking brake. The system is configured to apply indications of driver presence and absence to effect automatic control of the parking brake control valve, including automatically applying the parking brake upon an indication of driver absence and preventing release of the parking brake without an indication of driver presence. The driver absence indication can include the combination open driver's door and an empty driver's seat. The driver presence indication can include authorized driver identification through entry of a code or other authorization mechanism, as well as a requirement that the service brakes be applied.