Shutoff valve leaks are usually caused by loose packing nuts. A slight turn of a wrench often stops the leak.
By the DIY experts of The Family Handyman Magazine
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Fix a leaky shutoff valve
Tighten packing nut
Hold the valve body with one wrench and turn the packing nut clockwise with a second wrench.
Don’t panic. Here’s a simple fix that works more than half the time and doesn’t cost a dime to try. Many shutoff valves have a packing nut or packing material that surrounds the valve stem. The packing can shrink or get worn over time, causing water to leak out around the valve stem. This usually happens after someone has closed and opened the valve for a repair or for some other reason. The good news is that in most cases simply tightening the packing nut will fix the problem. Go easy, though. If you tighten it too much, the valve won’t turn. Quarter-turn shutoff valves don’t have a packing nut. If this type leaks around the stem, you’ll have to replace it.