Heater Control Switch "Hissing"

[sediver]

I recently noticed my heater control switch "leaking" on my 1995 GMC Jimmy 4.3L 4X4. I went the dealer and purchased a new switch and installed it and the leak remained the same. I thought it may be in the vacuum harness so I replaced that too and the leak was still there(between the harness and the switch, at least that's where it sounded like where it was coming from). I "jumpered" a short piece of hose between the Grey hose on the harness(which I believe is the vacuum supply coming from the engine since it is the only line with constant vacuum) and the Black hose(which when the heater control switch is in the "off" position it "connects" both the Grey and the Black hoses) to simulate the switch being in the "off" position and noticed I'm getting vacuum coming from the Brown hose on the harness. I checked the switch and found when the switch is in the "off" position the Brown hose is open to the atmosphere and that is were my leak is coming from. I'm sure I don't have a bad heater control switch since I got another one from the dealer and had them cross reference the VIN number to make sure I was getting the correct switch. I'm sorry if this seems confusing but it's the best that I can explain the problem. I really wish I had a vacuum diagram for the HVAC system to troubleshoot this problem but I'm almost sure the problem is at the other end of the Brown hose and feel there should probably be no vacuum at the brown hose when the switch is in the "off" position. Any thoughts ? Any idea what is at the other end of the Brown hose ? It's burried somewhere in the dash and I'd hate to remove the whole dash to look into it further without some expert advice as to what might be the problem.
Thanks for your time,
SEDIVER
Tucson, Arizona

[rlith]

I would look at the main vac diaphram that's under the center of the dash. These are known to go bad. Your actual "hiss" may be coming from there and the sound is traveling. The vac diaphrams are known to go bad.

[herkyhawki]

If the hissing really is coming from the heater control switch...disasseble the switch and put lots of thick silicone grease between the two plates that port the vacuum.

[DetroitMuscle]

My hiss sounds like it comes from below a trim piece under the dash on the driver side.

[rlith]

Originally Posted by DetroitMuscle

My hiss sounds like it comes from below a trim piece under the dash on the driver side.

Vaccum control unit (big fat diaphram)

[sediver]

I know its been a while since I posted this problem but I wanted to follow up and say that the problem ended up being one of the vacuum vent actuators behind the dash. Specifically, the "Center Outlet Duct Actuator." Kind of a bear to replace but it fixed my problem.