Abstract: An automatically operating air bleeding device for a liquid supply system, especially a fuel supply system for a diesel engine, comprises a tank having a fuel inlet, an upper air outlet valve, a lower fuel outlet valve, a pivotally mounted float and a pivotally mounted follower member resting on the float for effecting opening of the air outlet valve if the fuel in the tank falls to a first predetermined level due to the entry of air into the tank, and for closing the fuel outlet valve if the fuel falls to a second and lower predetermined level, and a magnet on the follower for actuating a switch for a warning device prior to the fuel lowering to said second level.