About two weeks ago our downstairs toilet started squeaking while the
tank refilled after every flush. The squeak got louder the past couple
of days and yesterday I noticed that water is leaking into the toilet
bowl. We have Hunter 528 anti siphon system in the tank. It looks like
this:
http://www.plumbingmart.com/lavellefillvalves.html

Should I replace the whole valve or is there an adjustment I can make
to bring the water level below the overflow pipe. I'm a newbie so
please be descriptive in your replies.

Thanks.