It is important to keep compressed air as dry as possible because
- moisture may cause air brakes to rust.
- humid air may cause incorrect readings on the pressure gauges.
- humid air is associated with low air pressure.
- moisture may freeze and cause brake failure. ✓
Answer: D. moisture may freeze and cause brake failure.
Keep compressed air dry because moisture can freeze in cold temperatures and may cause brake failure.
Why this is the answer
The handbook says moisture in the air brake system can freeze when it is cold and may lead to brake failure. It also explains that an alcohol evaporator is used in cold climates to mix with moisture or ice and reduce the chance of freezing. That points to freezing moisture as the reason to keep the air dry, rather than the other choices listed.
Why the other answers are wrong
- A. moisture may cause air brakes to rust. — The passages mention corrosive damage and sticking valves from moisture and oil, but they do not give rusting as the reason to keep compressed air dry.
- B. humid air may cause incorrect readings on the pressure gauges. — The passages do not say humid air causes wrong pressure-gauge readings; they focus on moisture freezing and on moisture and oil damaging components.
- C. humid air is associated with low air pressure. — The passages do not link humid air to low air pressure; they mention too much moisture and oil reducing air volume in the tanks, which is not the same claim.
Remember it
Think cold weather: keep water out so it cannot turn to ice in the brake system.
Official handbook: Official MTO Air Brake Handbook, Air Supply Subsystem – Supply or "wet" tank