Trailer Brake Control System

A vehicle brake system includes a hydraulic service brake system having a service brake circuit and a trailer brake control system. The trailer brake control system includes a trailer brake valve with a service brake demand input port and a trailer brake demand output port connected to a trailer brake control coupling. The output at the trailer brake demand output port is dependent on the pressure applied at the service brake demand input port. An electronic trailer brake control system has a solenoid control valve to connect the service brake demand input port to a source of pressurized hydraulic fluid to trigger an output at the trailer brake demand output port. An ECU is configured to actuate the control valve to apply the trailer brakes if it determines that the vehicle is at risk of jack-knifing. The system may also include a shuttle valve.