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)?