An automated analyzer for performing multiple diagnostic assays simultaneously includes multiple stations, or modules, in which discrete aspects of the assay are performed on fluid samples contained in reaction receptacles. Such a composition has improved stability on storage. A secondary closure device, also referred to as a second valve, moves independently of the first valve to control venting of the vapor space after nozzle shutoff through a bypass vent opening formed in the valve housing in response to at least one operating condition in the fuel tank outside of a chamber defined by the valve housing in which a main float moves.