The invention relates to a trailer comprising a frame, wheels mounted on the frame and provided with a wheel brake, a coupling (2) mounted on the frame for movement in the direction of travel and a transmission mechanism (5) for transmitting a movement of the coupling to the wheel brake system as a control movement and further provided with an auxiliary braking device comprising a pressure fluid motor (26) which engages onto the transmission mechanism and which is connected via a valve device (7) to a source of medium under pressure (8). The valve device comprises an electrically operated feed valve (24) and a variable reducing valve (22) which is connected to the coupling such that when the coupling is moved rearward a higher pressure is adjusted in the conduit to the pressure fluid motor.