I have a 2001 Cadillac Deville. Started over heating in February of this year.

What is your question?

I took it to to the repair shop. They pressure tested the cooling system. Found thermostat Gasket housing was leaking they replaced the thermostat and gasket. Filled system with proper amount of coolant to -35.bled air from system. That same day it over heated again brought it back they checked it and said it needed a new head gasket. Cost to repair would be 3,000 dollars so I tried Bar's Head gasket and cooling sealant in march it worked until October 17th. It overheated again I was About a mile from home it was steaming and I still drove it home. I talked to someone from Bar's company ask if I could use the head gasket sealer again they said I need to drain and flush the system real good and Then I could use it again and it might work again. How do I drain and flush the system? Any advice you can give me would be nice.