A combined fill and safety vent plug adapted to being mounted in a port in a pressure tank. A valve housing member is adapted to being mounted in a port in a pressure vessel. A fill valve is located in a fill valve socket in the valve housing member. A vent valve and an overflow system are also provided in the valve housing member. All of the functions of filling, venting, dispensing, and overflow protection are provided by elements contained in one single plug in one single port in a pressure vessel. For safety purposes, the vent valve dumps excess fluid generally laterally of the valve housing member.