Device for controlling a trailer-brake valve , connectable to the braking system of a trailer, for hydrostatic-driven towing vehicles which is provided with at least one variable displacement pump (P), comprising: - at least one adjustment line connectable to at least one of a delivery line of a work fluid under pressure and a discharge line and connectable to an adjustment device (C) for adjusting the displacement variation in the pump (P) of a towing vehicle, said adjustment line being adapted to convey the work fluid to the pump itself to vary the displacement thereof; - first piloting means operatively connectable to a trailer-brake valve to displace it from a home position to at least one braking position of the trailer, characterized by the fact that said first piloting means comprise - at least one accumulator of the work fluid, - at least one fluid operated cylinder inside which is defined a first chamber communicating with said adjustment line and a second chamber communicating with a thrust line which, in turn, communicates with said accumulator , wherein inside said cylinder is housed in a sliding manner at least one piston on which said first and said second chamber act on opposite sides, said piston being adapted to interact with said trailer-brake valve to displace it from the home position to the braking position as a result of a decrease in pressure occurring in said first chamber with respect to said second chamber , and characterized by the fact that it comprises at least one connecting channel interposed between said adjustment line and said thrust line and flow adjusting means arranged along said connecting channel adapted to allow the passage of the work fluid from said adjustment line to said thrust line and to choke the passage of the work fluid from said thrust line to said adjustment line , so as to define a pressure difference between them as a result of a drop in pressure along said adjustment line . - at least one adjustment line connectable to at least one of a delivery line of a work fluid under pressure and a discharge line and connectable to an adjustment device (C) for adjusting the displacement variation in the pump (P) of a towing vehicle, said adjustment line being adapted to convey the work fluid to the pump itself to vary the displacement thereof; - first piloting means operatively connectable to a trailer-brake valve to displace it from a home position to at least one braking position of the trailer, characterized by the fact that said first piloting means comprise - at least one accumulator of the work fluid, - at least one fluid operated cylinder inside which is defined a first chamber communicating with said adjustment line and a second chamber communicating with a thrust line which, in turn, communicates with said accumulator , wherein inside said cylinder is housed in a sliding manner at least one piston on which said first and said second chamber act on opposite sides, said piston being adapted to interact with said trailer-brake valve to displace it from the home position to the braking position as a result of a decrease in pressure occurring in said first chamber with respect to said second chamber , <EOS>