The Bendix+ Trailer System-Guard air dryer removes moisture and contaminates from the trailer air system. It is not intended to take the place of a dryer normally located on the power unit. It acts as a buffer to remove moisture during wet times and gives up water vapor during dry times. It protects the trailer air brake system when, for short periods of time, the trailer is pulled by vehicles without an air dryer or during times when the trailer is disconnected from the tractor.