Why should the trailer supply valve (the red button or valve) be closed before disconnecting the air lines during uncoupling?

Pick an answer

1. Closing it first sets the trailer brakes so the trailer stays in place once the tractor is disconnected
2. It is only relevant when coupling, not uncoupling
3. It should always be left open during uncoupling to allow for a faster disconnect afterward each time
4. It has no relationship to the trailer's braking during uncoupling

Correct!

Not quite. The correct answer is highlighted.