An auto bleed valve and method of bleeding a hydraulic system of an aircraft is accomplished by illuminating a plenum of a bleed valve using an LED oriented to illuminate a phototransistor when air is present in the plenum. A comparator circuit is used on an output of the phototransistor such that a predetermined voltage output corresponds to air detected in the plenum. An integrating circuit and a second comparator in series with the first comparator circuit is used to determine when a predetermined time period is met in which air is continuously detected in the plenum. Thereafter an air bleed solenoid can be used to vent the air upon a signal from the aircraft computer logic system.