If its that hard - you should roll it over with a moldboard plow - or a subsoiler can break up what sounds like hard pan.
You'll never get good root establishment with a light disking.
I've seen hardpan so hard that a plow bounces on top - Comes from working the upper 1-6" of soil with heavy machinery usually. If you can only overseed - Rye is one of the few seedlings that can help break up hardpan.
If its just dry and sod-filled - thats a whole other story.