CDL Combination Vehicles TestDriving Combination Vehicles SafelyHard
During a pre-trip inspection of a combination vehicle, a driver discovers that the air pressure in the trailer's service brake system is significantly lower than the tractor's. What is the most likely cause of this discrepancy?
- AA fully engaged parking brake on the trailer.
- BA leak in the trailer's air lines or brake chambers.
- CThe tractor's air compressor is malfunctioning and over-pressurizing the tractor system.
- DThe trailer's air tanks are overfilled, causing pressure relief.
Show answer & explanationAnswer & explanation
Correct answer: B. A leak in the trailer's air lines or brake chambers.
A significant difference in air pressure, with the trailer's system being lower, is a strong indicator of an air leak in the trailer's air lines, brake chambers, or connections. This leak prevents the trailer's air tanks from holding proper pressure.
Why the other options are wrong
- A. A fully engaged parking brake uses air pressure to release, but it wouldn't cause a low service brake system pressure on the trailer when disconnected or in normal operation.
- C. If the tractor's compressor was over-pressurizing, the tractor's gauges would show high pressure, but it wouldn't directly cause *low* pressure in the trailer system unless a protective valve activated, which is less likely than a leak.
- D. Overfilling air tanks would lead to the safety valve releasing air, but not a 'significantly lower' pressure than the tractor, unless there's an underlying leak.
Trailer Air Pressure Discrepancy
A lower air pressure in the trailer's service brake system compared to the tractor's often indicates an air leak in the trailer's air lines or brake components.
- Air brake systems rely on sealed lines.
- Leaks cause pressure drops.
- Inspect air lines and connections regularly.
Memory trick: Low air in trailer, a leak's telltale sound, check all around.