Do trailer brakes automatically lock when the air lines have been disconnected?

Air pressure within the air braking system of a tractor-trailer (or large truck) is what prevents the brakes from being applied. The pressure of the air pushes back a large spring at each wheel. The large spring will apply the brakes when there is no pressure in the system. When the driver steps on the brake pedal, air is released out of the system and the brakes are applied. When the driver's foot comes off the brake pedal, air is quickly pumped back into the system and the brakes are released. Disconnecting the air line has the same effect on the trailer's brakes. Air is released from the system, and the brakes are applied. It is not the brakes themselves that lock. Application of the brakes causes the wheels to lock.