A self-contained vent valve unit for a liquid system wherein liquid is to substantially fill a cavity and thereafter be pressurized in the cavity, the valve unit having a first valve arrangement adapted to be disposed in fluid communication with the cavity for venting air from the cavity during low pressure filling of the cavity with the liquid. The first valve arrangement is adapted to automatically close upon the pressurization of the liquid in the cavity to a first certain value thereof. The valve unit has a second valve arrangement also adapted to be disposed in fluid communication with the cavity for venting the liquid from the cavity when the pressure of the liquid reaches a second certain value thereof.