Abstract: A tractor-trailer brake system has a tractor engine brake which can be turned on to brake the tractor, and which operates a switch that can open a valve in the air line to the trailer brakes to allow simultaneous energization of these trailer brakes. A timer is interposed between the switch and the solenoid valve in the air line to periodically close this valve and allow cooling off of the trailer disk brakes for several seconds while the tractor engine brake remains on. The ratio of on time to off time of the solenoid valve during operation of the tractor engine is at least 10:1.