Method of operating an electrically controlled braking system

When the brake pedal is depressed a pressure sensor (20) indicates the pressure required to apply the brakes (6) and the electric control system (23) adjusts a valve system (26,27,29) accordingly to admit air to the brake cylinder. Should the control unit fail an emergency brake device (14) (conventional pressure failure device) monitors that no air has been admitted to the brake cylinder and through a valve (17) and a changeover valve (4) supplies the cylinder from the pneumatic reservoir (12).