A compressed-air braking and brakes locking system for trailers is provided, which permits to release the trailer brakes and to apply and lock the trailer brakes again when the trailer is separated from its tractor. The system includes a compressed air reservoir and an emergency relay valve on the trailer and a hand operated valve so arranged to put the control inlets of the emergency relay valve and of the brake unlocking device selectively in communication with the control circuits of the tractor or with the reservoir of the trailer.
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