Is it a self built system or is it a factory sealed unit? Do you have a reservoir and if so what make? If not do you have enough room to add one? That is usually the best way to at least trap the air there and keep it from going back into the cooling system. Otherwise you can get one of these
and some fittings in you don't have any spares and hold it into the highest position. I would then all a section of hose onto the drain port, hold my finger over it until the air got to that point and then let if out and cap it.
Or something like this Swiftech MCRES-Micro Hi-Flo Reservoir
and the air just sits at the top and it's not a problem.