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.

1. Regularly inspect and service Air / water separators.
2. Regularly add Menthol Hydrate (gas-line antifreeze) in the red trailer air line at the glad hands.  The antifreeze will find its way into tanks and other passages including valve systems. Extend the air line to put enough into the system. 
3. Seal the glad hands from the weather when the trailer is parked and not hooked up to a power unit.

                The least expensive part to replace is often overlooked, yet that part can affect safety of a vehicle in many ways.  I am talking about shock absorbers.  The main job of a shock absorber is to keep tires in contact with the road.  People associate shock absorbers with a smooth ride.  Hollywood shows cars bouncing along with worn shocks, and the scene is amusing.  Fleet managers won’t think about shock absorbers on trailers because nobody rides back there so “who cares”?  In a sudden stop at highway speeds if the trailer wheels start to hop is a jack-knife truck/trailer combination about to occur? Could worn shock absorbers be the root cause? 
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                An inspection of tire tread for scalping around the circumference is a good indication of worn shock absorbers. A good mechanic will be able to diagnose this. The attaching points of shocks are insulated with neoprene that over time wear and can be found by grabbing the shock and shaking.  Any movement at either end of the shock absorber indicates a loose mount, and time to replace the shock absorber.  Fluid will leak out of a shock absorber and render it useless.  What is most interesting is that many jurisdictions will accept a shock absorber for safety inspections if it is not broken or unattached.  No further inspection is required by law during safety inspections.  A worn shock absorber can affect steering and braking, but it will still meet minimum safety regulations if it is attached and in one piece.  A worn shock absorber will cause uneven tire wear and reduce tire life.  Replacing a pair of shock absorbers will cost less than half the price of one tire.  A pair of new shock absorbers will improve tire life and reduce wear on other suspension components. Still, these are often the most overlooked safety item on vehicles today.