95 Pontiac Transport mini van 3.8L heating system problems...?

I just recently replaced the thermostat, because the old one wasn't doing its job anymore. I noticed on top of the thermostat housing, there is like a bleeding valve. Can anyone tell me how that works?? The other thing is that, yesterday while i was driving cold started to come out, out of the vents. But, it stayed like that for few minutes, then warm air started to come out again. What could be the cause??
I appreciate anyones help..
thanks
 
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