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what you first should do is turn the water off to the house before the water is feed to that one valve that is giving you the trouble, then go open the hose bib on the out side and let the water drain out and relive its water pressure. Once this is done now you can try and loosen the little brass cap off, be careful bras is soft and can be miss formed easy under pressure from a steel wrench. You should be able to turn the cap with a wrench counter clock wise to remove and loosen.
Will I be abl
le Will I be abto re-use the screw shutoff if I get it off? I assume something is stripped inside. Is it possible to buy a replacement?
the brass cap you should be able to find a new replacement at the hardware store, if for some reasons the brass thread is miss formed then the valve will need to be replaced.