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The purpose of this research was to design, performance test, and model a plastic steering valve for a water hydraulic vehicle. All of the hydraulic functions of a Jacobsen Greens King VI lawn mower were converted from oil to water hydraulics. The power steering valve was the hardest component to replace since steering valves are not typically made of plastic or stainless steel. The original valve was coated using electroless nickel phosphate, but this was not sufficiently durable. Rust was found on the valve due to wear and incomplete coating after 20 hours of operation. After discovering rust in the original valve, it was decided to build a plastic valve. The new plastic steering valve was a simple rotary valve constructed of polycarbonate. The valve was a jerk steer design where 35 of steering wheel rotation was required to reach full lock. Valve characteristics such as flow gain and pressure sensitivity, leakage, and deflection were determined for this plastic steering valve.