Dumbest reservoir and air questions ever?

[Brian R. Hamilton]

Never built a water-cooled PC before and the terms involved are so general that searching for answers was mostly useless. Why can't a piece of vertically oriented clear tubing on a T-fitting be used as a simple reservoir and to trap air as it passes by thru the T-fitting? Also, how do reservoirs work when they aren't the highest point in the system? Why doesn't the water above them just flow into the reservoir and fill the reservoir to the top, leaving any air in the system trapped at the highest point again?