Thanks for posting the pictures, I was trying to draw on mine but my program is limited.

The red line is the return line. Restrict it a little and some water and any air will flow through the smaller hose (yellow) into the overflow. On the underside of the overflow container is a hose that connects to the heat exchanger circuit so you don't have to worry about it going dry. It will replenish itself from the bottom of the overflow if necessary. (stock setup)

Les