Blazer vacuum operated vent selector switch

I have a 2003 Chevy Blazer 4x4. Last year I replaced the vacuum operated air vent selector switch, since it was leaking (hissing sound from switch, could not select the window setting, only the dash vent pushed air through). Since last April I have replaced the same part three times. They all work for about 2-4weeks, than the hissing sound starts and the selector switch stops working. When the last switch failed it was vacuum tested and it was confirmed leaking. The hissing sound comming from the switch is the loudest when the engine is idle, when accelerating the sound goes away. Someone suggested that the switch might be drawing in oil from the transfer case and the oil is ruining the rubber diaphram... When I took the switch apart it had grease inside and maybe some oil. Are these switches supposed to be lublicated or dry? I tried to clean the rubber diaphram and put it back in service, but it still leaking... The control panel (includes the heat, fan and ac/vent selectors) came from the dealer and were supposedly new. Any ideas why this switch keeps failing?

Also, could someone post a vacuum lines diagram for this vehicle. Thanks...

Just saw an update in another post on this transfer case switch and the HVAC at the end of the posts:
http://www.automotiveforums.com/vbulletin/showthread.php?t=572914

Thanks. Looks like that is what the problem is. I looked under the hood to find this transfer case vacuum switch, however cannot locate it. Which part of the engine compartment is this switch? Do I need any special tools to replace this part?

Meanwhile cleaned out the rubber diaphragm in the HVAC Mode Switch, put the switch back in the vehicle and it is operational right now.