Abstract: A tank for a fluid system which is pressurized in an operative mode and depressurized in an inoperative mode, the tank having a pressure vessel enclosing a chamber and a substantially annular port in the vessel communicating with the chamber; a mount for securing the vessel in fluid transferring communication with the fluid system through the port with the port disposed in substantially upwardly facing relation; and a substantially spherical flow control element captured in the chamber of the vessel and dimensioned gravitationally to seat in the annular port to allow passage of fluid through the port when the fluid system is in the operative mode, to vent pressure from the chamber upon depressurization of the fluid system and to seal the port when the fluid system is in the inoperative mode.