According to grodenhiATgmailDOTcom :
Shouldn't the float move up evenly (as the water does).

I consider the float sticking and water hammer two different
issues. If you address the latter without the former (ie:
cushioning the pipe where it bangs), you might end up with
the thing not banging anymore, just flooding the place...

It may be that the float has built up some hard water deposits, and
if it's in the slightest bit tilted while the water level rises,
it binds.

Wiping the column and the inside of the ring with some CLR
(or even hot vinegar) might help.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.