I used the schwaben coolant purge tool. The one you hookup to an air compressor. You can visually see the hoses fully collapse. I use like 90-100psi. Not sure if this fully gets air out, but it seems alright.

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Have a full bucket of coolant and then after you create the suction you open another valve and only coolant gets sucked back in with a hose you dunk in a bucket of coolant