Abstract: A vessel for a fluid, the vessel having a filling opening and a cap removably mounted on the vessel for closing the filling opening, the filling opening being defined by a tubular conduit having a vent port located in its wall, the cap including an elongate sealing member which is axially insertable into the conduit, the sealing member carrying a circumferentially extending main seal for providing sealing engagement between axially extending opposed faces of the conduit wall and the sealing member, the sealing member being axially movably mounted on the vessel for axial movement between a first axially static position whereat the main seal resides at an inner location relative to the vent port to sealingly isolate the vent port from the interior of the vessel and to a second axially static position whereat the main seal resides at an outer location relative to the vent port to permit direct fluid communication between the vent port and the interior of the vessel.