A valve intended to disconnect, in specific circumstances, the braking of the front wheels, and/or of a trailer whose braking system is connected through a trailer braking valve, for the braking system of a farm tractor or similar vehicle which comprises two master cylinders, operated by two separate braking pedals, directly controlling the braking of the rear wheels, and controlling the braking of the front wheels and/or the braking device of the trailer through the disconnection valve. The disconnection valve includes a body, a first and second inlet connections connected to the two master cylinders, and a delivery connection connected to a braking circuit of the front wheels and/or to the trailer brake valve. The valve body has a bore wherein is installed one movable piston subjected to the action of pressure applied to the first inlet connection.