This has always been my experience as well with doing anything to the cooling system. Takes weeks of daily driving and burping sessions to get things back in order. You can burp and burp and still it runs hotter than normal until whatever air or voo doo works its way out of the cooling system...
^^^^^^ This. Had the same issues first time I replaced cooling system bits OP. Got one of the linked funnels and its never been an issue since. Still takes some effort to get the air out, but the above tool makes it much easier to see what's going on.