antisiphon valve

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.
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snipped-for-privacy@yahoo.com wrote:

Goodness. Scroll down for the service/adjustment procedure. And there is a 5yr warranty./
My guess is the "main valve" is worn/dirty.
Jim
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It's 6 years old. Warranty is no good. I saw the instructions. I was wondering on how to get rid of the squeaking and also if I should replace the whole valve or not. Thanks for your reply.
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For 9 bucks don't fool with it. Replace it.
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wrote:

something to incite more hand wringing.
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Check whether the valve shuts off the water completely by lightly lifting the float. If it doesn't, then there is probably a replaceable valve seal. The common valve seals are easily available at Lowes or Home Depot, or any plumbing supply place. You might need to contact the manufacturer for this one to find a supplier.
As mentioned by others, there is an adjustment, but if the seal leaks, adjusting it won't help. If the water rises, and the float rises, but stops with the water still rising, that is another sign of a seal problem.
Bob
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