The disclosure relates to a valve for a fuel system having a body with at least one inlet and one outlet, the inlet fluidly connected to a pressurised fuel source in use. A shuttle is mounted within the body, the shuttle having a cavity of fixed volume and movable between a first position where fluid is permitted to flow through the inlet and is prevented from flowing through the outlet and a second position where fluid is prevented from flowing through the inlet and is permitted to flow through the outlet. A piston is configured to engage the fluid within the shuttle cavity to move the shuttle between the first and second position. A biasing mechanism biases the shuttle towards the first position and where the shuttle moves towards the second position when the fluid within the shuttle reaches a critical pressure.