A compression fitting that drips under the sink is usually loose, overtightened, or sealing on a sleeve that was already crushed onto the pipe. Dry it, find exactly where the water starts, and snug the nut a little at a time. If that does not stop it, shut off the water and redo the joint with a new sleeve and nut on a clean, square pipe end. SharkBite’s instructions for a compression stop valve: hand-tight until snug, then 3/4 turn with a wrench, and do not overtighten.
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If the old stop valve is the problem: a new quarter-turn angle stop, 5/8-inch compression inlet by 3/8-inch outlet (our stop valves by pipe type guide matches the inlet to copper, PEX or CPVC). Check the Eastman 10738LF angle stop on Amazon
Find Exactly Where It Leaks
Dry everything with a paper towel, turn the water on, and watch. Under a sink the water usually starts at one of these spots, and each has a different fix:
- Where the stop valve meets the pipe from the wall or floor: the inlet compression nut and sleeve. This page is about this joint and the next one.
- Where the supply tube or connector meets the valve outlet: the outlet compression nut. A braided connector has its own seal built into the nut end, not a separate brass sleeve, so if a gentle snug does not stop it, replace the connector.
- Around the valve handle or stem: not a compression leak. The valve itself is leaking; replace the valve.
- Higher up, at the faucet or the drain: see kitchen sink leaking underneath.
Step 1: Snug It
Hold the valve body with one wrench so it does not twist the pipe, and turn the compression nut with a second wrench, a small amount at a time. Dry it and check. SharkBite’s installation numbers give you the scale: a new stop valve goes on hand-tight until snug plus 3/4 turn, and the supply connector hand-tight plus 1/4 turn. A joint that needs far more than that to stop dripping has a sleeve problem, and cranking harder only crushes the sleeve further. Be gentle on chrome trim; see will channel locks scratch chrome.
Step 2: Redo the Joint
If a careful snug does not stop the drip, take the joint apart. The brass sleeve (also called a ferrule or olive) seals by being squeezed into the fitting, so once it has been tightened it is shaped to that joint and should not be reused.
- Shut off the water at the main and open a nearby faucet to drain the line.
- Take off the nut and the valve. The old sleeve usually stays stuck on the pipe. If you have spare pipe, cut behind it; if the stub is short, pull it off with a sleeve puller. Our guide to removing a stuck compression ferrule shows both.
- Prep the pipe end. SharkBite says to cut the copper stub squarely, then clean and deburr it with emery cloth and a deburr and gauge tool. A scratch or dent where the sleeve sits can leak again.
- Use the right sleeve. Compression sleeves are sized by the outside diameter of the tube: the common 1/2-inch copper stub measures 5/8 inch outside. If you are replacing the valve, use the nut and sleeve that come with it; if you are keeping it, use a 5/8-inch OD sleeve and nut that match the valve’s brand rather than mixing brands; compression ferrule sizing explains both points.
- Assemble in SharkBite’s order: slide on the nut, then the sleeve, then push the valve on until it bottoms out. Hold the valve tight against the pipe, thread the nut on by hand until snug, then give it 3/4 turn with a wrench. Do not overtighten.
- Reconnect the supply connector hand-tight until snug, then 1/4 turn, and open the valve and the main slowly. Check again with a dry paper towel.
No tape on the sleeve. The seal is the sleeve pressed between the nut and the fitting, not the threads, so tape on the threads does not fix a compression leak; a new sleeve on a clean pipe end does. If an old valve keeps weeping around its handle, or you are redoing the joint anyway, a new quarter-turn valve is the better repair; see our under-sink shut-off valve picks.
I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.
Parts That Fix It
Which of these you need depends on the joint. Each was sold or shipped by Amazon, new and in stock, when we checked on September 24, 2026; details come from each Amazon listing.
- A new angle stop, if the valve is old or leaks at the stem. The Eastman 10738LF quarter-turn angle stop is 5/8-inch OD compression (1/2-inch nominal copper) by 3/8-inch OD compression, with Teflon seats and a brass ball, per the listing. It is the top pick in our under-sink shut-off valve guide.
- A sleeve puller, for a short stub. The compression sleeve puller for 1/2-inch compression fittings pulls the nut and sleeve off 1/2-inch copper compression joints, per the listing.
When to Call a Licensed Plumber
- The main shut-off will not close, or you cannot find it. See how to shut off your water main.
- The stub coming out of the wall is too short, corroded, kinked or loose in the wall.
- The pipe is galvanized steel or a type you cannot identify.
- A new sleeve on a clean, square pipe end still leaks.
- The water is coming from where the sink meets the countertop, not from a fitting. Our undermount sink adhesive guide covers resealing that joint and when to call a pro.
FAQ
How do you fix a leaking compression fitting?
Snug the nut a little at a time and check for drips; that is the first try. If it still leaks, take the joint apart and check the sleeve (ferrule): a sleeve that has been tightened before is shaped to that joint, so a leaking one usually needs a new sleeve and nut, or a new valve, fitted on a clean, square, deburred pipe end.
How tight should a compression fitting be?
SharkBite’s instructions for a compression stop valve are hand-tight until snug, then 3/4 turn with a wrench. For the supply connector from the stop to the faucet or toilet, hand-tight until snug, then 1/4 turn. Both warn against overtightening.
Can you reuse a compression sleeve?
It is not a good idea. A compression sleeve seals by being squeezed into the fitting, so once it has been tightened it is shaped to that one joint. A leaking joint that has been taken apart should get a new sleeve and nut.
Should you use Teflon tape on a compression fitting?
The seal in a compression joint is the sleeve pressed between the nut and the fitting, not the threads. Tape on the threads does not help; if a joint leaks, fix the sleeve rather than wrapping the threads.
What size compression sleeve do I need?
Match the outside diameter of the tube. The common 1/2-inch copper supply stub under a sink measures 5/8 inch on the outside and takes a 5/8-inch sleeve; the 3/8-inch supply tube to a faucet takes a 3/8-inch sleeve.
Why Trust Plumbing Picks
Plumbing Picks is written by a licensed Georgia plumber — journeyman license #JP507711 — and the license is verifiable through the Georgia Secretary of State’s license verification portal. The installation steps on this page follow the SharkBite source listed below, and product details come from the Amazon listing or the maker, not from hands-on testing. Nobody pays to be included here. Where a link goes to Amazon we may earn a commission at no extra cost to you. More on who writes this and how we pick.
Sources
Bottom Line
Dry it and find the exact drip. Snug the compression nut a little at a time with the valve held still. If it still leaks, shut off the water, take the joint apart, and fit a new sleeve and nut on a clean, square, deburred pipe end: hand-tight plus 3/4 turn for the stop valve and hand-tight plus 1/4 turn for the supply connector, per SharkBite. No tape on the sleeve, and replace an old valve rather than fighting it.