A main valve (16) includes a product pressure inlet (18), an outlet (20), a movable closure member and a pressurized head volume above the closure member. Product pressure relief flow through the main valve is controlled by a modulating, pressure operated pilot relief valve (14) that includes a piston chamber (36) with a reciprocally mounted pilot piston (42). The chamber is in fluid communication with the pressurized head volume and with the outlet and inlet of the main valve. An inlet valve (92) controls flow from the inlet of the main valve into the chamber and is actuated by the pilot piston. The pilot piston includes first and second surfaces (46) and effective areas DELTA A exposed to the pressure in the piston chamber and an end isolated from the piston chamber and in fluid communication with the main valve inlet.