A pressure relief valve has a moveable piston disposed in a body between inlet and outlet ports in the body. A volumetrically variable enclosed chamber is formed within the body with a portion of the chamber being defined by an upper surface of the piston. At least one aperture extends through the head of the piston providing a fluid passageway between an inlet port of the valve and the enclosed chamber. Fluid captured within the enclosed chamber permits initial rapid movement of the piston head to an open, or relieved, position in the event of a sharp increase in system pressure and provides a hydraulic cushion whereby impact shock of the head against internal valve components is attenuated.