Just unscrew at the black threads and replace with a 1/2” mip brass plug, like this below. Look at the cone washer in the coupling nut. If it’s deformed, you may need to replace that. Easy peasy.
The Everbilt Pipe Square Head Plug is the perfect solution when you need to close the end of a copper, brass or iron pipe. The square head design makes installation easy. The plug features durable lead-free,
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