I am a new home owner and am glad to have found this site as this is my first time owning a pool also. The attached picture shows the plumbing of an existing water line we've used to fill the pool when it gets low. Yesterday the plastic cap on the right began leaking when I turned the black cap on to fill the pool up. I'd like to replace the cap obviously. Is this common hardware found readily at Lowe's/HD? Does it have a "name"? If I've got the black cap turned down, then in theory no water should be coming through the line so I shouldn't have to turn the water off to the house to repair, yes? Any ideas what specific function the cap serves other than closing the line?