Reduced pressure zone assembly consists of two independently operating, spring loaded cam-check valves with a hydraulically operated differential pressure relief valve located between and below the cam-checks, required test cocks and optional inlet and outlet resilient seat shutoff valves. When normal flow exists, both checks, called the zone, is at least 2 PSI lower than the inlet pressure. The differential pressure relief valve is closed during the normal flow. The cam-checks include a stainless steel spring and cam-arm, rubber faced disc and a replaceable seat. It is used to protect against high hazard (toxic) fluids in water services to industrial plants, hospitals, morgues, mortuaries, and chemical plants.