A fuel tank pressure relief valve for relieving a pressure difference between the interior pressure of a fuel tank and an exterior pressure external to the fuel tank, comprising: first and second sealing surfaces for mating to form an openable seal between the interior and exterior of the fuel tank; a housing provided with the first sealing surface; a first plunger; a second plunger; and a venting port, wherein the first plunger is provided with the second sealing surface, wherein the first plunger is flexibly sealed to the housing by a first membrane providing a first diaphragm area to open the seal formed between the first and second sealing surfaces when the interior pressure is above the exterior pressure by more than a first threshold differential pressure; wherein the second plunger is flexibly sealed to the housing by a second membrane providing a second diaphragm area that is greater than the first diaphragm area; and wherein the first plunger is biased towards a closed configuration of the first and second sealing surfaces by a resiliently deformable member to maintain the seal formed between the first and second sealing surfaces in a closed configuration when the interior pressure is not above the exterior pressure by more than the first threshold differential pressure and is not lower than the exterior pressure by more than a second threshold differential pressure, characterized in that the second plunger is engaged with the first plunger to bias the first plunger towards an open configuration of the first and second sealing surfaces when the interior pressure is lower than the exterior pressure to open the seal formed between the first and second sealing surfaces when the interior pressure is lower than the exterior pressure by more than the second threshold differential pressure. first and second sealing surfaces for mating to form an openable seal between the interior and exterior of the fuel tank; a housing provided with the first sealing surface; a first plunger; a second plunger; and a venting port, wherein the first plunger is provided with the second sealing surface, wherein the first plunger is flexibly sealed to the housing by a first membrane providing a first diaphragm area to open the seal formed between the first and second sealing surfaces when the interior pressure is above the exterior pressure by more than a first threshold differential pressure; wherein the second plunger is flexibly sealed to the housing by a second membrane providing a second diaphragm area that is greater than the first diaphragm area; and wherein the first plunger is biased towards a closed configuration of the first and second sealing surfaces by a resiliently deformable member to maintain the seal formed between the first and second sealing surfaces in a closed configuration when the interior pressure is not above the exterior pressure by more than the first threshold differential pressure and is not lower than the exterior pressure by more than a second threshold differential pressure, characterized in that the second plunger is engaged with the first plunger to bias the first plunger towards an open configuration of the first and second sealing surfaces when the interior pressure is lower than the exterior pressure to open the seal formed between the first and second sealing surfaces when the interior pressure is lower than the exterior pressure by more than the second threshold differential pressure.