Relay valves are divided into differential relay valves and relay valves. The differential relay valve seems to have an additional port. Used for emergency braking; I don't know if you are talking about the follow-up bridge, it has no handbrake. The hand brake acts on the parking cavity of the rear axle of the automobile, that is to say, it acts on the air chamber of the drive axle, and the parking brake is released by pulling the hand brake. (i.e. inflating the parking cavity to overcome the mechanical spring force of the parking cavity). Then you can drive. ...
A little knowledge gives a lot of advice.