The FSI Siphon Stop is a siphon prevention device used on submersible pumps for aboveground storage tanks. The system is simply designed to break a vacuum utilizing a check valve. It works in conjunction with the submersible pump “tank port” to allow air to enter the line in a negative pressure situation. It will prevent the tanks contents from siphoning through the submersible pump. Thermal expansion is relieved via the standard STP pressure relieving system. It does not effect the operation of mechanical leak detectors.