On the main water line to my house, there is a black gasket that seems to be leaking. What is that black nut on top? Have you seen this kind of thing before? I want to know if I can just tighten it with a wrench or if I need to turn of water line and replace it.

leaking rubber gasket

  • Do you have an irrigation system? From what I can see that's part of a backflow preventer. Also if you have winter where you are, water pipes not drained for winter would need to be buried below the frostline, which these clearly are not.
    – Tyson
    Aug 27, 2016 at 0:32
  • @Tyson yes, that pipe is right next to the irrigation system, so maybe I'm wrong and this isn't an incoming pipe. Aug 27, 2016 at 1:25
  • It's appears from the pic it's a test port on the irrigation systems backflow preventer. Since you didn't know what the part is, I'm guessing you have a company or person that maintains the system for you, call them.
    – Tyson
    Aug 27, 2016 at 1:28
  • @tyson thanks! you helped me realize the problem. It's a test cock on a backflow preventer and there is an easy way to turn it off. Aug 27, 2016 at 1:35

1 Answer 1


The picture turns out to be a picture of a test cock on a backflow preventer. The black plug is just cosmetic. One the side there is a way to use a flathead screwdriver and turn off the test cock (in my case, it had been slightly open).

  • Man, this site is good!
    – bib
    Aug 27, 2016 at 1:59
  • Black plug is likely more than cosmetic, it would be a cap to stop any dirt and debris getting in.
    – D-on
    Aug 27, 2016 at 7:58

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