Not quite. There are two fluid paths to and from the reservoir - a larger orifice that supplies fluid during the pumping stroke, and a smaller one that returns fluid when the lever is released. That small orifice is frequently the culprit when brakes drag - dried brake fluid or tiny flecks of schmutz can easily clog it in part because it is so small. When you pull the lever halfway you are creating a pathway for fluid (or air bubbles in this instance) from the reservoir to the volume of fluid in the lines through this smaller orifice. The air will want to rise, as long as it has somewhere to rise to, and vent to the atmosphere (again, the reservoir). If it has nowhere to rise, it stays in the lines.
