The Bendix+ TR-2 and TR-3 valves are normally open, pilot-operated, inverting, on-off, two-way valves. As control pressure is reduced - to the point that the piston return spring or springs overcomes the force on the control piston - the valve will deliver full supply pressure at the delivery port. The TR-2 valve was used primarily in early Bendix+ DD-3 safety actuator control systems. It applied air pressure to the emergency diaphragm of the DD-3 actuators from a protected reservoir as control pressure is released from the lock mechanism. \nThe Bendix Trailer Line-Lock valve (most commonly referring to the Bendix+ TR-3 inversion valve or similar shut-off valves) is a component used to manage air signals to specific trailer systems, primarily lift axles. It --locks-- or disrupts the air signal to prevent the service brakes from applying to an axle that is currently in the raised position.