Pam,

   The "self bleeding" feature takes place over a few cycles of heating and
cooling. Air collects under the pressure cap and is forced out as the system
heats up. Inversely, coolant is drawn in as cooling occurs.
   The ring draws off trapped air (as far as I can tell)  from the tops of
the heads and both sides of the thermostat to be directed to the reservoir
where it awaits it's expulsion mention above.
    Ideally, when you check it at each visit to the gas station, you want to
see  ZERO air in the reservoir and the level in the overflow tank between
min and max.
Ken Lewis
http://neksiwel.20m.com/