Techie,
I assume you mean air in the brake hydraulic line. In that case, when brakes applied the air would be compressed. When hyd brake press released the air would tend to expand and voila - a dragging brake. Also heat from the brake pack would tend to make the air expand in the line. In an older system with lockout deboosters - a la B707 - said air certainly cannot escape, not too sure about modern systems.
How'd I do, coach???