How do you take the seal off of a Mansfield toilet #160?

I have this same toilet. When you look in the toilet tank you'll see a white "tower"-like structure. On the top of this you'll see a tube entering it through a disk shaped structure. After turning off the water supply and flushing the toilet to drain the tank, you simply unscrew this disk and then you can lift up the tower. The seal is just below this. It's easy to remove & replace. I already did this, but it hasn't solved my problem with it running! Can anyone help me with this problem?

Jane R.
December 2008