Automatic reservoir bleed valve

Abstract

An automatic valve for bleeding air from liquid or with reversal of the principal elements, vice-versa. In an embodiment for bleeding free air from a hydraulic fluid reservoir or the like, a capillary extends through a movable piston in series with an orifice to differentiate between hydraulic fluid and air due to differences in the pressure drop across the capillary which results from differences in the viscosity of the fluids attempting to pass therethrough. When air, having relatively low viscosity, flows through the capillary, the differential pressure across the capillary and, accordingly, across the piston, is low and the valve remains open to bleed the air. When hydraulic fluid attempts to pass through the capillary, however, the differential pressure thereacross due to the fluid's relatively high viscosity is sufficient to move the piston and close the valve. In the preferred embodiment, the bleed valve may also utilize a second movable piston to positively seal the valve at low reservoir pressure, thus preventing the valve from leaking fluid when the hydraulic system is inactive.