When coupling twin trailers, you should release the dolly brakes by `____`

A Opening the air tank petcock

Explaination

Release dolly brakes by opening the air tank petcock (valve), or if the dolly has spring brakes, use the dolly parking brake control.

B Dropping the trailer onto the fifth wheel

C Letting the air out of the brake line

D Pushing the air release valve on the dash

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When coupling twin trailers, you should release the dolly brakes by ____