Fluid flow valves which include air release or air purge features are well known.  These valves are generally located within liquid conduits and are designed to insure the release of air or other gases to avoid air locks which otherwise interfere with the flow of liquid through the conduit. 
One known air purge valve construction is disclosed in U.S. Pat.  No.  4,770,201.  In that patent, a valve is provided which includes a housing having a defined fluid throughflow aperture, a valve seat formed in the housing and bounding the aperture; and a flexible closure membrane or diaphragm secured at one end of the housing and adapted to be biased under fluid pressure against the valve seat so as to seal the aperture.  The other end of the diaphragm is connected to a float in the housing so that movement in one direction results in progressive detachment of the membrane from the valve seat so as to open the aperture.  Movement of the float in an opposite direction allows the membrane to become sealingly biased against the valve seat. 
In some of the known air purge type valves, it has been found that the valves tend to be "blown" closed by the velocity of the air escaping through the valve, thus significantly reducing their effectiveness.  Attempts have been made to address this problem by designing floats with low aerodynamic drag coefficients.  This change has been only partially successful in that the valves still tend to be blown closed at relatively low pressure differential across the valve (for example, less than 10 p.s.i.).  Another approach has been to place a shield in the valve to direct the air around the float.  Again, while this has been somewhat successful, the valve still blows closed when the pressure differential across the valve exceeds approximately 14 p.s.i.  Moreover, additional components are required to implement this solution, thus increasing cost and complexity.