Usually you can just steer it stop to stop a few times and it will purge the air. If that dose not do it then loosen one line at a time right at the steering cylinder. Have someone hold the steering wheel against the stop in the direction that makes oil flow out the loosen line. When the oil it flowing without any air bubbles tighten the line. Then do the other direction.
If you still have trouble that you may have a system relief valve issue. So of the combines have a separate one from the steering. Yours could be bad or set wrong.