This is on a 1998 Bravada. Water pump took a dump so it was time to install all my upgrades. Gaskets, spider, timing chain, balancer.
Everything went pretty well. During disassembly, the hoses attached to this nipple were total garbage and rotted away. The hose went up to a check valve, the t'd off that to a tiny nylon hard line and t'd off that same fitting again to my fender. Didn't see any end it would attach to. All of it just fell apart the second I moved it. So it wasn't functioning before, so I just capped it for now.

 

That nipple would normally attach up to the vacuum system lines near the brake booster. The line it's attached to allows air to vent from the head back into the upper air plenum. I'm not 100% sure, but I think it might be part of the vacuum supply that controls parts of the heater and 4x4 system to function.

 

Interesting. I didn't notice any problems with blend doors, in fact anything so far.
I'd have to double check the check valve fitting. It seems like it would only release pressure, not the other way for vacuum.