Solenoid Function Issue

[barf]

2009 ProStar; there is a 4 valve solenoid located inside left frame just ahead of the air dryer. The rear valve is for the air horn. The middle 2 are connected to some other kind of valve (in the picture) that goes to one valve. I don't know what these valves are for. My problem and question is that this pictured 3 way valve is leaking air. What is it's function and can it be replaced with just a 3 way valve or is it some kind of one-way thing? It's an audible leak and drains my tanks to about 50psi in an hour then stops leaking.



This was sort of discussed in another thread: http://rawze.com/forums/showthread.php?tid=3002 but I thought it should be in "International Trucks" section.