Commercial Driving Air Brakes 2 — Questions and Answers
Question 1: What is the purpose of drain valves on air brake tanks?
- To add air to the system
- To remove accumulated water and oil from the air tanks that could cause brake failure (Correct answer)
- To check tire pressure
- To release parking brakes
Correct answer: To remove accumulated water and oil from the air tanks that could cause brake failure
Air compressors introduce moisture and oil into the air tanks. Draining these contaminants prevents corrosion, ice formation in cold weather, and degraded brake performance.
Question 2: How should you test the low air pressure warning?
- Drive the vehicle until brakes fail
- With the engine off, pump the brake pedal to reduce pressure and verify the warning activates before 60 PSI (Correct answer)
- Ask a mechanic to test it
- Low air warnings cannot be tested
Correct answer: With the engine off, pump the brake pedal to reduce pressure and verify the warning activates before 60 PSI
With the engine off, repeatedly pressing the brake pedal reduces air pressure. The warning light and/or buzzer should activate before pressure drops below 60 PSI.
Question 3: What is a slack adjuster?
- A tool for adjusting mirrors
- A lever mechanism that adjusts the distance between the brake shoes and drum to compensate for wear (Correct answer)
- A seatbelt tensioner
- A steering alignment tool
Correct answer: A lever mechanism that adjusts the distance between the brake shoes and drum to compensate for wear
Slack adjusters connect the pushrod to the brake camshaft and compensate for brake lining wear, keeping the brake adjustment within proper operating limits.
Question 4: What is the air brake applied pressure test procedure?
- Drive and brake repeatedly
- Build pressure to operating level, turn off engine, apply the brake, and observe pressure drop — it should not exceed 3 PSI in one minute for single vehicles (Correct answer)
- Listen for air leaks only
- Check the brake fluid level
Correct answer: Build pressure to operating level, turn off engine, apply the brake, and observe pressure drop — it should not exceed 3 PSI in one minute for single vehicles
With full pressure and the engine off, applying the brake and monitoring pressure drop identifies air leaks in the service brake system. More than 3 PSI drop per minute for single vehicles indicates excessive leakage.
Question 5: What is the function of the relay valve in an air brake system?
- To relay messages to the driver
- To speed up brake application and release on trailers by using local air supply near the brakes (Correct answer)
- To control the turn signals
- To power the air horn
Correct answer: To speed up brake application and release on trailers by using local air supply near the brakes
Relay valves reduce brake lag on trailers by allowing air to flow from a local tank near the brakes rather than traveling all the way from the tractor, speeding up application and release.
Question 6: During a pre-trip inspection, how much free play should a manual slack adjuster have?
- No more than 1 inch (Correct answer)
- No more than 2 inches
- Any amount is acceptable
- Exactly 3 inches
Correct answer: No more than 1 inch
Manual slack adjusters should have no more than 1 inch of free play. Excessive free play indicates the brakes need adjustment and may not provide adequate stopping power.
Commercial Driving Air Brakes 2
What is the purpose of drain valves on air brake tanks?