A flow through fluid pressure regulator includes a spring biased plunger which is normally biased away from a plunger seat in the direction of fluid flow to provide maximum flow under normal conditions. In the event of a pressure surge, the movement of the plunger in the opposite direction to decrease flow is controlled by a damper. Specifically, the plunger is formed with a radially outwardly facing groove or damper chamber at its downstream end, and into which a resilient O-ring is loosely fitted. This downstream end of the plunger moves within a reduced diameter bore portion of the regulator housing which, in turn, communicates with a diaphragm chamber formed in a larger diameter portion of the housing. The diaphragm chamber surrounds the plunger and is further defined by a flexible diaphragm fixed at an inner radial edge to the plunger and at an outer radial edge to a stationary sleeve in the housing. The outer diameters of the radially outwardly extending flanges which form the O-ring groove or damper chamber, and the loose fit of the O-ring within the groove, permit water to flow past the groove and into the diaphragm chamber in a controlled manner. In an alternative arrangement, the damper chamber and O-ring are located in the housing surrounding the plunger.
