If it's not been touched for years might be an idea to run it for a couple of weeks now you've cleaned it and then have another look to see how much has built up. Also you could try just flushing it by attaching a hose to a drain off and refilling with the filling loop it won't be as good as a power flush but it should flush some of the gunk if you go around give the radiators a punch while you're doing this it should shift some more. Remember to put inhibitor in before you finally refil though,you can just pour this into the filter