Abstract: A fluid leak detector mechanism having a main fluid tank, an expansion tank for receiving fluid therefrom as the fluid becomes heated and expands, and a sensing chamber operatively coupled with the main tank. A signalling mechanism signals to an operator when the level of fluid in the sensing chamber has dropped significantly to alert the operator of the presence of a leak. An oil return valve is operatively coupled between the main tank and the expansion tank and has an open position that allows fluid to flow back to the main tank from the expansion tank and a closed position that blocks oil from flowing from the expansion tank to the main tank. A temperature sensor senses the temperature of fluid. A control mechanism shifts the oil return valve to its open position when the temperature sensor detects a significant drop in fluid temperature, thereby allowing the fluid levels to be reset with changes in fluid temperature.