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Describe what the trailer air supply control does.

A It is used to supply the trailer with air, shut the air off, and apply the trailer emergency brakes.

The trailer air supply control on contemporary vehicles is a red eight-sided knob on the tractor's dashboard. "Pull to exhaust...Push to supply" or something along those lines may be written on the knob. When you turn the knob, air will flow through the emergency air lines on the trailer, charging the trailer air brake system. When you pull the knob out, the air flow is cut off, and the trailer's emergency brakes activate.