how semi truck trailers don’t just roll away when disconnected from a semi truck
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In the US a trailer has two airlines that connect it to the tractor.
One airline supplies air to open the brakes. There are very powerful springs in the brake canisters that keep the brakes applied when there is no air pressure.
The other airline supplies air to the braking side of the brake, you know, when you press on the brake pedal. Or when you pull on the “Johnny bar” which will apply brakes to the trailer.
With both airlines disconnected from the trailer the strong springs will close the brakes and keep the wheels from turning.
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