New radiators shouldn't actually need to be flushed. Your brand-new radiator or system won't need to be "broken in." Finding cold patches on a new radiator, though, may indicate that your system already included some nasty muck or rust. It's possible that this then travelled to your new radiator. Your system will then require flushing. There can be an issue with the radiator if you have power cleansed the system within the last three years.