CDL General Knowledge TestCombination Vehicles (if applicable)Medium
You are driving a combination vehicle and notice that the trailer brakes are locking up frequently, especially when braking lightly. What is a common cause of this issue?
- AThe trailer's brakes are out of adjustment or too aggressive for its load.
- BThe tractor's ABS system is malfunctioning, sending incorrect signals to the trailer.
- CThe emergency air line has a slow leak, causing partial brake application.
- DThe trailer is overloaded, causing excessive weight transfer to the rear axle.
Show answer & explanationAnswer & explanation
Correct answer: A. The trailer's brakes are out of adjustment or too aggressive for its load.
If trailer brakes lock up easily, especially with light braking, it often indicates that they are over-adjusted (too tight) or are simply too aggressive for the current load, causing them to apply disproportionately to the tractor brakes.
Why the other options are wrong
- B. ABS malfunction usually causes loss of braking or erratic braking, not consistently locking up with light pressure.
- C. A slow leak in the emergency line would likely cause the spring brakes to apply fully or intermittently, not just light braking lock-up.
- D. An overloaded trailer would typically require more braking effort, not less, to lock up the brakes.
Trailer Brake Lock-up (Light Braking)
A condition where a combination vehicle's trailer brakes lock up prematurely or with minimal pedal pressure, often due to improper adjustment or brake aggressiveness.
- Indicates brakes are too tight or over-adjusted.
- Can lead to trailer skids and loss of control.
- Requires immediate inspection and adjustment.
Memory trick: Locking up lightly means the brakes are too tightly.