Unusual toilet tank flush valve repair?

Most toilet tanks have a flapper flush valve. But I have one with an unusual cylinder valve, like this:
http://www.mansfieldplumbing.com/v3/support04-03.html
It has developed a leak, so I need to repair or replace it.
Does anyone know how to take it apart? The tank sits 3 inches below a shelf that's firmly attached to the wall, so it's hard to see and investigate.
Would I need to separate the tank from the toilet to replace this with a flapper-style drain valve? Or just to replace the seal (if I can even find such part)?
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It seems to me that regardless of whether you repair or replace, you are going to need more than 3" of space to work.
You might not even be able to get the tank off the bowl with only 3" of clearance.
"firmly attached to the wall" could mean many things, but it doesn't mean the shelf can't be taken off. Do you know how it is attached?
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It is far more easier to remove the shelf from the wall than to take apart a toilet tank just for the sake of leaving the shelf where it is. And yes, they sell replacement parts for that type of flapper. These guys are pretty good. http://www.plumbingpartsdepot.com / They have all drawings and schematics online for a lot of toilets. Chances are you can find the OEM part for your toilet.
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