What accumulates at the bottom of the supply reservoir in an air brake system?
- Brake fluid
- Water droplets
- Sludge ✓
- Dust particles
Answer: C. Sludge
Grey sludge accumulates at the bottom of the supply reservoir.
Why this is the answer
The handbook explains that air from the compressor usually carries some water vapour, which condenses into liquid water. Small amounts of oil and carbon particles also pass through, and these mix with the water to form a grey sludge. Because most contaminants settle in the supply reservoir, it is often called the wet tank. If sludge is allowed to build up, it can enter other braking system components, so the reservoirs have drain valves and manufacturers usually recommend draining them daily.
Why the other answers are wrong
- A. Brake fluid — Brake fluid is not described as part of this air brake system; the passage talks about compressed air, water, oil, and carbon particles forming sludge.
- B. Water droplets — Water vapour does condense into liquid water, but the passage says oil and other contaminants mix with that water to make grey sludge, so water droplets alone are not the accumulated substance described.
- D. Dust particles — The passage names oil and carbon particles as contaminants from the compressor, not dust particles.
Remember it
Picture the wet tank: water plus oil and carbon settles into grey muck, so drain the reservoirs regularly.
Official handbook: ICBC Driving Commercial Vehicles, 8 Air brakes – Foot valve, p. 153