Leaking hose bib

I have one of those long, freeze-proof hose bibs (outdoor faucet). It has a small round plastic thing on top of it behind the valve stem. I assume it is an extra safety precaution in case of water freezing in the hose. Recently, water has begun to poor out of that plastic area whenever the faucet is opened. Is this something that can be repaired or does the entire hose bib need replaced?
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rile wrote:

You are probably talking about a vacuum breaker hose bib. Since we don't know the manufacturer, you'll need something like this: http://preview.tinyurl.com/25nsag Or try looking around that site after you know the brand of faucet.
--
Grandpa

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That is exactly what I am talking about. Now if I can only find the brand and where to get one locally. Thanks for the help.
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