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What is a shut-off valve?
A.  A device that prevents air from escaping an air brake system
B.  The ignition switch
C.  An object connected to the spring brake system
D.  A coupling device
Shut-off valves are used in air supply lines to control the passage of air from one trailer to another. The rear shut-off valve should always be closed to prevent air from leaving the braking system.

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