Why is it unsafe to release the trailer parking brakes and rely only on the tractor's parking brakes when parking a combination vehicle?

Pick an answer

1. The trailer's spring brakes only work when connected to shore power at a terminal or yard
2. The tractor's brakes alone may not be enough to hold the full combined weight of the tractor and trailer, especially on a grade
3. Trailer parking brakes have no holding function of their own
4. It is always safe as long as the trailer is empty

Correct!

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