Abstract: In a one way non-return valve comprising a cylindrical duct, a cone in said duct with its apex pointing in the direction of flow and provided with a plurality of apertures, and a membrane lying on the downstream surface of the cone and clamped at the outer circumference, the membrane having a central aperture, the improvement which comprises employing a cone having an apex angle of about 140.degree. to 160.degree., the membrane comprising a disk of elastomeric material having a central circular aperture which engages the hollow cone under bias. Advantageously the apertures in the cone are a series of concentrically disposed, round or long-oval apertures, the hollow cone has an apex angle of about 154.degree. and the total area of the apertures in the cone is about twice as great as the area of the aperture in the membrane.