What accumulates at the bottom of the supply reservoir in an air brake system?
- Water droplets
- Dust particles
- Brake fluid
- Sludge ✓
Answer: D. Sludge
A mixture of moisture and oil that forms sludge accumulates at the bottom of the supply tank.
Why this is the answer
The supply tank is the first tank the compressed air enters, and it catches most of the moisture and oil that drops out of the air, which is why it is also called the wet tank. The handbook says this collected moisture and oil mixture can form a sludge that moves into other parts of the system, damages seals and makes brake valves stick. The drain valve sits at the bottom of each air-tank, so that is where the collected material gathers and gets drained off.
Why the other answers are wrong
- A. Water droplets — Moisture is only part of what the tank collects; the handbook describes that moisture mixing with oil to form sludge, so water droplets alone is not the answer it gives.
- B. Dust particles — Dust is never mentioned as something the supply tank collects, since the passages talk about moisture and oil dropping out of the compressed air.
- C. Brake fluid — The system stores and manages compressed air, and what drains from the tanks is moisture and oil, not brake fluid.
Remember it
Think of the wet tank as the settling point: whatever the air drops out ends up down at the drain valve, so drain the supply tank first and watch what comes out.
Official handbook: Official MTO Air Brake Handbook, Air Supply Subsystem – Supply or "wet" tank