A self-venting drain valve is rotatably and slidably mounted to a filter assembly through an internally threaded base. The drain valve comprises a homogenously formed one-piece unitary valve body including a tubular valve stem portion and a valve knob portion. The valve stem portion includes a wall member defining an internal passageway provided with axially extending air vent openings through which ambient air can enter a filter housing through the internal passageway, and drain holes through which water is drained from the filter housing and the internal passageway. The air vent openings are formed through an upper portion of the wall member and are open to a top edge thereof, while the drain holes are formed through the lower portion of the wall member. The liquid drains from the filter housing through the internal passageway and the drain holes, then exits into the valve knob portion.