A switch in particular for a master cylinder unit, for opening and/or closing an electrical circuit, for example, for switching on the brake lights, comprises a casing suitable for being housed in a compartment of the cylinder body, a first electrical contact fixed firmly to the casing, a movable element slidable in the casing, a second electrical contact associated with the movable element, and a resilient element for keeping the movable element constantly urged against the piston of the master cylinder unit.The switch forms a switch box which is produced separately and can be associated with the cylinder body in the compartment provided therein.