Wanted to share my experience with the G-max and Water hammer after the tank fills.
I have a 1 1/2 year old Toto Drake. Recently the fill valve would oscillate (open/closed) when it was done filling creating a nasty water hammer.
The water hammer was magnified by the fact the water supply is 3/4 pipes to this bathroom. The water pressure in the shower is very powerful, but the water hammer was pretty scary at this pressure!
After reading some of the posts here, I realized that maybe what I needed was a new cap.
I found one here:
The site also says that it was available at Lowes. Since Toto isn't very popular here on the East coast I didn't think I had a good chance of finding it there.
To my surprise I found one at my local Lowes. The packaging was a little dusty though.
Less than $5 and the problem was solved.
The diaphragm in the old cap was a little misshapen. I think we have a high chlorine level in our water.
You may have solved the problem, but when you said the pressure in the shower is "pretty powerful", it makes me wonder of the cause of the problem was too much pressure. Have you put a gauge on your water supply? If you have much more than 60-65 psi, you need to install a pressure reducer valve and an expansion tank. Too much pressure could be what damaged the fill valve, and it could damage the new one in time as well.