Vacuum Check Valve 1 / 4" (6mm) These one way check valves are used to prevent boost pressure from entering any unwanted sources and throwing check engine lights ie: Break master cylinder, MAP sensors, evap sensors...etc Always use one for the break master cylinder, horror stories about blown master cylinders, check engine lights, loosing breaking power, are not good! Maximum Pressure: 3,45bar at 21°C Cracking Pressure: 0,03bar Temperature Range: -51° bis 93°C