What should the low air pressure warning device do on a properly functioning doubles or triples combination?

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1. Activate before pressure drops low enough to risk the spring brakes applying, just as on a single-trailer combination
2. Warn the driver only about the rearmost trailer's pressure, ignoring the others
3. Only activate once the tractor's engine has been shut off
4. Stay off no matter how low the pressure drops, since trailers have their own separate warning systems built into each unit

Correct!

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