17 GMC burb vacuum line HELP!!

[66mustangguy]

okay I have been searching and searching, but have not found a good post on 71 GM suburban vac lines.

I am trying to figure out what the three vacuum lines into the cabin are for. I got the burban in pieces and I have no idea and have not found anything useful on where they go or are used for.

all the heating from what I can tell are managed by wire and the AC is roof AC and is not part of the heating system. I reached under the dash to try and see what it was connected to but didnt have much luck. been search a long time but no luck on these.

please help!!