mansfield valve replacement?

[lumangoy]

I have a mansfield toilet. Water is continously flowing to the bowl. If I turn the water off, the tank will be empty. I can't figure out how it's going to the bowl.

The black seal inside the tank seems to be disintegrating. After googling, I found out that I need to take the tank out to replace the mansfield valve. I've taken the tank out, but can't seem to take the valve out.

Is there an easy way to take it out?

or should I be looking at someplace else for the leak?

thanks.