There has to be an air cushion in the expansion tank, or it can't do its job. I'm not sure how they maintain an air cushion, as air is normally absorbed by the water since it is in contact, and hopefully, extracted as air is what allows corrosion in a system. A bladder type tank has no air contact (until it needs replacement) and will allow an air extraction device to work properly and maintain low air (and therefore oxygen which allows things to rust) in the water. I'm not a pro, but I'd replace that expansion tank with a bladder type.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
The only thing I would change on the monoflow system is, to install at least 2 automatic air vents at the highest points of your heating system or if that is not possible, you can install 2 ball valves with a hose bib in between.
This will allow you to isolate each side of the valves and bleed the air from the hose bib with a high flow rate. Shut off one side and then bleed from the hose bib (attach a standard garden hose to hose bib). Re-open valve and shut off the other side and re-bleed. It may take a few tries to get it all out. You can have more control by throttling the valves.