cooling system on 95 baretta

I drained a friend's radiator on her 95 baretta. Is there a relief valve of some kind to purge any air out of the system, when re-filling? I can't seem to be able to fill the radiator(from the overflow resevoir). The low coolant light stays on and the engine is running hotter than normal. Why won't the radiator fill from the overflow resevoir?