Pressure relief systems are designed with automatic diurnal emission control and overpressure relief safety valves built into the cap of the fill. The overpressure relief safety valves are engineered to release pressure above 1 psi. The high-flow vacuum valves allow fresh air to enter the fuel tank to replace the fuel being consumed by the engine. Other components of a pressure relief system are designed to ensure the tank fills to the proper level every time without spit-back or well-back. 

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