The 6" Watts 990 Reduced Pressure Zone (RPZ) was produced from 1993-1997. The body was constructed of fused epoxy coated ductile iron. Check assemblies uses a toggle linkage mechanism design. Check springs are contained when the cover is removed. Spring tension must be relieved to repair the checks. Seats are removable but a special seat removal tool is required. The relief valve assembly is constructed of bronze and threaded onto the body. An external sensing line is used.