A mechanism for opening or closing a shut off valve, the mechanism having a lever mounted on a spindle to which the valve is attached for mounting the valve from a normally open position to a closed position. A trigger mechanism including the lever and a biasing spring which when activated, rotates the lever, spindle and valve. A retainer mechanism ordinarily holds the trigger mechanism in a cocked position corresponding to the open position of the valve. The float, when subjected to water, releases the retainer mechanism permitting the trigger mechanism to close the valve.