The trailer hand valve should not be used to hold an unattended trailer because
- air brakes are more effective.
- an air brake system leak could cause the brakes to fail. ✓
- a trailer should never be left unattended.
- it is not strong enough.
Answer: B. an air brake system leak could cause the brakes to fail.
Do not use the trailer hand valve to hold an unattended trailer because an air brake system leak could let air drain away and cause the brakes to release.
Why this is the answer
The handbook warns never to use a trailer hand valve to hold an unattended unit. Over time, air can drain away and the brakes may release. That is the risk behind the correct answer: if the air system leaks or loses pressure, trailer brakes held only by the hand valve can let go. The handbook also says the hand valve should not be used for parking; for an unattended trailer, it says to apply the tractor parking brakes, close the trailer supply valve, and block the trailer wheels.
Why the other answers are wrong
- A. air brakes are more effective. — Air brakes being more effective does not explain the warning about holding an unattended trailer with the hand valve; the concern is air draining away and the brakes releasing.
- C. a trailer should never be left unattended. — The passages do not say a trailer can never be left unattended; they say not to rely on the hand valve to hold it when it is unattended.
- D. it is not strong enough. — The handbook does not say the hand valve is too weak; it warns that air can escape over time and the brakes may release.
Remember it
Treat the hand valve as a trailer-brake control, not a parking brake; if you walk away, air can leak off.