CDL Combination Vehicles TestDriving Combination Vehicles SafelyMedium
You are driving a combination vehicle and notice that the air pressure gauge for the trailer's service brakes is significantly lower than the tractor's. What does this discrepancy most likely indicate?
- AThis is normal operation; trailers always operate at lower air pressure.
- BThe tractor's air gauge is faulty and providing an incorrect reading.
- CThe trailer's braking system is operating more efficiently than the tractor's.
- DThere is a leak in the trailer's air supply line or brake system.
Show answer & explanationAnswer & explanation
Correct answer: D. There is a leak in the trailer's air supply line or brake system.
A significant difference in air pressure between the tractor and trailer, with the trailer's pressure being lower, is a strong indicator of an air leak in the trailer's air supply line, glad hand connection, or brake system components.
Why the other options are wrong
- A. This is incorrect; both tractor and trailer air systems should equalize pressure when connected and charged, indicating a problem if there's a significant difference.
- B. While possible, a faulty tractor gauge is less likely than an air leak when there's a discrepancy specifically with the trailer's pressure.
- C. Braking efficiency is not directly indicated by lower static air pressure; lower pressure suggests a problem, not better performance.
Air Pressure Discrepancy
A significant difference in air pressure between the tractor and trailer (lower in the trailer) indicates an air leak in the trailer's system, requiring immediate inspection.
- Tractor and trailer pressures should equalize.
- Lower trailer pressure suggests a leak.
- Leaks compromise braking effectiveness.
Memory trick: Low trailer air? Check for a leak, beware!