Truck manufacturers include water separators in the air lines after the compressor and before the Air brake, and air suspension components. This is a good first step to supplying clean, dry air to brake and suspension components. The trailer brake system is not protected the same way. In fact, there is generally an air tank in the trailer storing compressed air. Most air tanks have a valve on the bottom to drain water from the system. Some manufacturers recommend draining the tank daily. When temperatures dip below freezing this is not enough because moisture is already frozen in the air lines. As an Inspector, I have seen many Air brake systems frozen and not providing any brake application whatsoever to the trailer brakes. This is dangerous. Equally dangerous is an air suspension system that is frozen. Mechanics know that collapsed air suspensions are a symptom of inoperative air brakes.
All problems associated with frozen air lines could be eliminated by following a few simple procedures.