As part of the whole vehicle air brake check, why do you also check that the trailer's brakes hold when the tractor protection valve is tested?

Pick an answer

1. It measures the trailer's total cargo weight indirectly
2. It confirms the trailer's emergency brakes will apply correctly if the air supply to the trailer is ever lost
3. This test is only relevant for trailers built before the mid-1990s, an assumption that doesn't hold up with tested
4. It only checks the trailer's turn signals, not its brakes

Correct!

Not quite. The correct answer is highlighted.