It is important to keep compressed air as dry as possible because
- humid air is associated with low air pressure.
- humid air may cause incorrect readings on the pressure gauges.
- moisture may cause air brakes to rust.
- moisture may freeze and cause brake failure. ✓
Answer: D. moisture may freeze and cause brake failure.
You want the compressed air kept as dry as possible because moisture may freeze and cause brake failure.
Why this is the answer
The handbook's description of alcohol evaporators and injectors shows why dryness matters: these devices add alcohol vapor to the air system so it mixes with any moisture present and works like anti-freeze, lowering the freezing point of that moisture. If water in the system were not treated this way, it could freeze. The reason the handbook gives for adding the alcohol is freezing protection, which points to frozen moisture as the danger being guarded against, not pressure loss or gauge error.
Why the other answers are wrong
- A. humid air is associated with low air pressure. — Nothing in the passages links humid air to low air pressure; the passages discuss moisture and freezing, not pressure levels.
- B. humid air may cause incorrect readings on the pressure gauges. — The passages never mention pressure gauges giving wrong readings because of moisture.
- C. moisture may cause air brakes to rust. — Rust is not mentioned anywhere in the passages; the concern described is moisture freezing, which is why alcohol is used as an anti-freeze.
Remember it
Think of the alcohol injector as anti-freeze for the air system: the enemy is ice in the lines, not rust or a bad gauge.