I've always refilled via the overflow tank, and it's been fine. One thing I do is to add the proper amount of straight (undiluted) coolant to the empty system first. Then I add an equal amount of distilled water. It's likely you won't be able to get enough water into the system right away. After a few days, you'll notice that the air bubbles have worked their way out, and then you can just add more water to achieve the proper mix.

- dave