Did you ever figure this out? I'm having the same problem with my A/S Hamilton 1-piece. It also has the "Hunter" flush valve.

Is this what you have? I found this info on terrylove.com

Here is a link that might be useful: hunter flush valve

Apparently, one piece toilet flush valves need a special wrench that you use with a ratchet. If you buy a new flush valve, it comes with the wrench (glorified piece of plastic) and is not too difficult to get the flush valve out. I bought the replacement at http://www.plumbingpartsdepot.com/, who was conveniently down the street from me. They do mail order, too. All you need is the toilet type - a four digit number embossed in the tank starting with a 2.

Unfortunately, after replacing everything, the toilet still leaks. The plumbing parts guys gave me some ideas to try to fix it, but it looks like there's a defect in the porcelain that isn't allowing a proper seal. I haven't had the energy to have another go at it, but pretty soon I'll be trying a nail file and plumbers putty. Wish me luck!