Abstract: A locking device engageable in mating relationship with a glad hand brake line coupler of a semi-trailer disconnected from its tractor is disclosed. A depressible lock mechanism supported by a housing of the locking device is connected to a plunger which is extended into an air passage opening in a heavy rubber grommet of the glad hand coupler when the lock mechanism is depressed. Once the plunger is extended, it can only be retracted by means of a key which unlocks the depressible lock mechanism. When the locking device is matingly engaged with the glad hand coupler, it is prevented from vertical displacement from the glad hand connector by engagement of tongues and corresponding tongue receiving recesses of the glad hand and the locking device. The locking device is prevented from lateral or rotational displacement with respect to the glad hand coupler by the engagement of the plunger with the inside walls of the air passage opening in the rubber grommet.