The valve manifold on the side of the 5 ton shifter actually has 4 connections, air from main system in, two air control lines to the transfer case out and a 4th that is the dump port (open) when the system shifts, the hisssss when you go from forward to reverse. On our two trucks this port is just open, but for the fording kits, this may be connected to a vent system, not sure. Just make sure the valves and the shift cylinder are working correctly, if not, the transfer case could go BANG!!