[ECUPirate]

You're on the right track, but having the airtrap at the top makes things MUCH easier. Air will rise to the highest point it can. There can be multiple 'high points' in a system. For instance, if a waterblock is positioned so that both hoses point downwards, air will accumulate in the block regardless of where the airtrap is (absent any water flow). The waterblock has formed an airtrap.

Of course, good waterflow through the system can fairly easily flush out the waterblock, but if the heater core is trapping air, that could be more difficult.

In my system, the rad is at the bottom (barbs at top), the waterblock is above that, the pump higher still, and the res is at the top. Air in the rad rises up through all of the parts and into the res, even w/out flow. Granted, small bubbles required the water flow to flush out... Bleeding this system was a breeze. I poured water in the res, and watched it flow down the system, filling up all of the parts!