Abstract: A reduced pressure zone backflow preventer comprising a body 12 entirely made of plastic material, formed with an inlet port (14) accommodating an inlet check valve assembly 120 and an outlet port accommodating an outlet check valve assembly 122. The check valve assemblies extend along a coaxial axis coinciding with a longitudinal axis of the body. A central space 60 formed between the check valves is accessible through a service opening in the body. A relief valve assembly 70 is received within the central space and a drain port extends from the space. The relief assembly is responsive to pressure differential between the inlet port and the space by a flow passage extending between the inlet port and an upper side of a pressure diaphragm of the relief valve assembly.