Here are a couple of links to help.
This one, Plumbing has a Link to an article on the 2nd page titled, 'Fixing A Leaky Toilet'. (You can search the Page for the work 'toilet'.) It has a reference to the ball cock valve.
Another good site is How Stuff Works. On it, is another link to toilets (and how they work.) Of course, it's a general description of How Stuff Works so be mindful of that.
Happy fixing, and let us know how you made out. My best to ya and hope this helps.
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