This leak comes from a cut or nicked 18-PA Tank Valve Oring. These Orings screw in and out over threads every time you install and remove your air tank. It is the most common cause of a leak, and the Oring replaced most often. Be sure to always have extra.
Check your Chronograph Speed / Pressure, and input pressure and adjust as needed.
To adjust the markerís velocity, use a ⅛ inch Allen wrench in the holes of the velocity adjuster. The velocity adjuster is located on the right side of the Upper Receiver. The Piston Assembly has a built in Pressure Relief Valve. This assembly is factory preset, and should not be adjusted or disassembled. It will vent in the case of over pressurization to prevent damage and failure of other components.