Abstract: A pneumatic tractor-trailer brake system by which application of the trailer brakes and the tractor brakes occurs simultaneously to guard against jack-knifing. This is accomplished by connecting the pressure chamber of the relay valve on the trailer and the adjacent end of the service line with the air pressure tank on the trailer by opening a normally closed solenoid-actuated valve as a consequence of initial depression of the brake pedal. Energization of the solenoid to open this valve results from closure of the energizing circuit for the stop light on the trailer. The resulting immediate response of the relay valve pressurizes the entire trailer brake system, including the service line, sufficiently to take up any slack in the trailer brakes no later than that occurs at the tractor brakes. At the instant the brakes begin to grip, a fluid pressure actuated electric switch opens the solenoid energizing circuit.