An exhaust after-treatment system adapted to supply a metered amount of diesel exhaust fluid to a mixing zone in an exhaust pipe coupled to a diesel engine, the system comprising a diesel exhaust fluid tank venting control unit including a fill-limit valve module adapted to receive diesel exhaust fluid from a diesel exhaust fluid tank, a breather-valve module exposed to the atmosphere, and a vapor-transfer module interposed between and coupled in fluid communication to each of the fill-limit valve module and the breather-valve module, wherein the breather-valve module includes an interior chamber and a semi-permeable membrane (431MLF, 431MHF) having a topside exposed to atmospheric air admitted into the interior chamber and an underside exposed to fluid vapor associated with diesel exhaust fluid in the diesel exhaust fluid tank and fluid vapor conducted through vapor-transfer module, wherein the breather-valve module is formed to include a membrane receiver that is arranged to open downwardly toward an interior chamber formed in the vapor-transfer module and adapted to communicate with an interior region of the diesel exhaust fluid tank via the interior chamber of the vapor-transfer module when the diesel exhaust tank venting control unit is mounted in an aperture formed in a wall of the diesel exhaust fluid tank and the semi-permeable membrane (431MLF, 431MHF) is mounted in the membrane receiver, and wherein the membrane receiver comprises a small membrane-receiving cavity arranged to lie in spaced-apart relation to the interior chamber of the vapor-transfer module and a relatively larger membrane-receiving cavity arranged to lie between the small membrane-receiving cavity and the interior chamber of the vapor-transfer module and the membrane (431MLF, 431MHF) is mounted in only one of the small and relatively larger membrane-receiving cavities (43RS, 43RL). a diesel exhaust fluid tank venting control unit including a fill-limit valve module adapted to receive diesel exhaust fluid from a diesel exhaust fluid tank, a breather-valve module exposed to the atmosphere, and a vapor-transfer module interposed between and coupled in fluid communication to each of the fill-limit valve module and the breather-valve module, wherein the breather-valve module includes an interior chamber and a semi-permeable membrane (431MLF, 431MHF) having a topside exposed to atmospheric air admitted into the interior chamber and an underside exposed to fluid vapor associated with diesel exhaust fluid in the diesel exhaust fluid tank and fluid vapor conducted through vapor-transfer module, wherein the breather-valve module is formed to include a membrane receiver that is arranged to open downwardly toward an interior chamber formed in the vapor-transfer module and adapted to communicate with an interior region of the diesel exhaust fluid tank via the interior chamber of the vapor-transfer module when the diesel exhaust tank venting control unit is mounted in an aperture formed in a wall of the diesel exhaust fluid tank and the semi-permeable membrane (431MLF, 431MHF) is mounted in the membrane receiver, and wherein the membrane receiver comprises a small membrane-receiving cavity arranged to lie in spaced-apart relation to the interior chamber of the vapor-transfer module and a relatively larger membrane-receiving cavity arranged to lie between the small membrane-receiving cavity and the interior chamber of the vapor-transfer module and the membrane (431MLF, 431MHF) is mounted in only one of the small and relatively larger membrane-receiving cavities (43RS, 43RL).