1978 Chevy Silverado

Turns out this thing:

Passenger side on the firewall.

… is the “vacuum reservoir” for the AC unit.  The AC is mostly gone from the engine compartment.  I’m leaving it on the firewall, but I disconnected it from the vacuum line.

EDIT: Also called the “accumulator ball”.  Ok, disconnecting that turned out to be a bad idea.  Even with no AC, you still need that line to operate the vent controls.  Without it, the various selection controls do nothing.  Took me 7 months to figure that out, sadly.

EDIT 2014 Update:
I now have aVintage Air system that does away with all the vacuum components. So I have now removed the accumulator.