It's been a couple years since I pulled the TBI motor out of my firebird and now I am working on installing it into an S-10. I am trying to remember what the fitting in the back of the manifold behind the TBI hooks to. Attached is a picture with the fitting I am talking about circled in red.
Thanks for the help.

 

A guess- since i'm a TPI guy, but hose size looks like it goes to master break cylinder.

 

Huh?

Brake master cylinder isn't vacuum-operated.

 

I have one of those and mine is the main vacuum supply for the cruise control.
I was concerned about the missing vacuum ball and the hoses on top of the cruise control unit. I was looking for a vacuum leak and in the process found it. I don't use cruise control so I found that with a small piece of hose and a screw in the end of it.

A large and small hose fitting are on top of the cruise control unit .That goes on the large one and the other to the vacuum ball.
This link shows you mine.

http://imageshack.us/a/img20/6540/hyzj.jpg

 

Opps, meant to say Break Booster (connects to master).