Device for controlling the braking of a trailer, comprising: - at least one body (2, 3) associable with a trailer and in which is formed at least one seat having at least one braking port, communicating with a control line connectable to the braking system of a towing vehicle to receive a work fluid under pressure, and at least one delivery port, communicating with a delivery line connectable to the braking system of the trailer; - at least one distributor housed sliding inside said seat and movable between at least one braking position, in which said braking port is communicating with said delivery port, and at least one balance position, in which said braking port is isolated from said delivery port; - at least one driving channel communicating with said delivery port and acting on said distributor to push it away from the braking position; - command means acting on said distributor, on the opposite side of said driving channel, and adapted to exert a force of variable intensity depending on the load of said trailer; characterized by the fact that said command means comprise: - at least a first command element connected in a fluid-operated manner to said control line and adapted to displace between a rest position and a thrust position by effect of the pressure increase of the work fluid along said control line; - at least one connecting element adapted to transfer the force from the first command element to said distributor and interposed between them; - adjustment means of the force transmitted by said connecting element to said distributor, where said adjustment means are connectable to the trailer and are moveable between a position of maximum load, in which the force transmitted to said distributor is maximum, and a position of minimum load, in which the force transmitted to said distributor is minimum. - at least one body (2, 3) associable with a trailer and in which is formed at least one seat having at least one braking port, communicating with a control line connectable to the braking system of a towing vehicle to receive a work fluid under pressure, and at least one delivery port, communicating with a delivery line connectable to the braking system of the trailer; - at least one distributor housed sliding inside said seat and movable between at least one braking position, in which said braking port is communicating with said delivery port, and at least one balance position, in which said braking port is isolated from said delivery port; - at least one driving channel communicating with said delivery port and acting on said distributor to push it away from the braking position; - command means acting on said distributor, on the opposite side of said driving channel, and adapted to exert a force of variable intensity depending on the load of said trailer; - at least a first command element connected in a fluid-operated manner to said control line and adapted to displace between a rest position and a thrust position by effect of the pressure increase of the work fluid along said control line; - at least one connecting element adapted to transfer the force from the first command element to said distributor and interposed between them; - adjustment means of the force transmitted by said connecting element to said distributor, where said adjustment means are connectable to the trailer and are moveable between a position of maximum load, in which the force transmitted to said distributor is maximum, and a position of minimum load, in which the force transmitted to said distributor is minimum.