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An automatic air deflation valve is provided for use with a sphygmomanometer type blood pressure measuring instrument. The valve comprises a housing having an air flow channel extending through the longitudinal extent thereof and at least one air deflation port extending outwardly from said channel. The port includes a deformable diaphragm supported only on its outer edges and having a central aperture extending through the thickness thereof. The diaphragm is adapted to deform in accordance with the air pressure applied against it from said channel to automatically adjust the size and shape of said aperture, thus producing a constant air deflation rate therethrough.