OK, let's go through this slowly: an air valve can be of various shapes. A common one on car tyre, sometimes referred to as "French Valve" sticks out of the tyre, and can be covered by a cap... The cap is commonly topped by a narrower cylinder, that can be referred to as "nipple". In English the term "tyre valve's air nipple" often refers to the whole air valve, t.e the cylinder which sticks out of the tyre. In Slovak I would translate simply as "Čiapočka vzduchového ventilu" which really refers to the air valve cap, but those in the know would clear about the meaning...


NOTE: Another common tyre valve type used on racing bicycles is referred to as Schroeder Valve, which is narrower with a simple round cap (no nipple there :-)
See drawing of the French valve attached...

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Note added at 3 days 4 hrs 58 mins (2006-01-02 01:00:12 GMT) Post-grading
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Your translation is good, mine just gave a bit more detail. Depending on the level of technical detail needed in the document it might have been important, but from the context it sounded as a descriptive brochure only, nit a technical instruction sheet. Anyway, I've just noticed I forgot to put that little image online - it's there now at http://www.devteam.com.au/images/airvalve.gif
Stastny a vesely Novy Rok!