What is the role of the relay valve, often found near the rear brakes on a tractor or trailer?

Pick an answer

1. It speeds up brake application and release by supplying air from a nearby tank instead of routing it all the way from the treadle valve
2. It only functions when the parking brake is set
3. It replaces the need for brake chambers at the rear axle entirely, since the relay valve applies the brakes on its own without them
4. It controls the headlights when brakes are applied

Correct!

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