This is how I was able to get my airtrap in the optimal setup for my case...
Basically, before I had the airtrap, air just stayed in my radiator, and I could hear the sloshing of the air & water. now the inlet for the airtrap is about 1/4" lower than the outlet of the radiator, but you can't hear any air in the radiator anymore.
One thing that I learned, is that if you sit there, and pinch the tubing, and then let it go (while the pump is on), you can force more air out of your radiators faster than just letting it sit there and run.
If you can get away with it, tip your system around while it's running, see if you can get anymore air outa that rad.