I think you are referring to the height control valve. This has 3 ports. The lower port connects to the air compressor, the center port connects to the air spring and the top port has a short hose and is the vent port. The hydraulic line does not connect to any of this and is probably routed close to the airlines causing the confusion. It is normal for the vent hose to leak air as the suspension moves so you may need to be more specific. Does the rear suspension deflate rapidly after you turn the engine off? After you dump air, for leveling for example, does the suspension rise quickly after you start the engine? Provided the compressor can keep up with the air loss you can put off having this checked until it is convenient.
John and Mary Knight
2015 Ventana 4311, 2015 Cadillac SRX