Step-by-step fix
Step 1: Shut off the water and set up the area
- Remove stored items from under the sink so you can work clearly.
- Place a towel and small bucket under the supply line connections.
- Turn the hot and cold shutoff valves clockwise until they stop.
- Open the faucet to relieve pressure and let any remaining water drain out.
If it works: Water flow from the faucet slows to a stop and the area under the sink is protected.
If it doesn’t: If water keeps running, the shutoff valve may not be closing fully. Try closing it gently again. If it still does not stop, shut off the home's water before continuing.
Stop if:- The shutoff valve will not close.
- The valve body leaks when you turn it.
- You cannot stop water flow at the sink.
Step 2: Confirm the replacement line matches
- Look at the old supply line and note whether it serves the hot or cold side.
- Measure the old line length from connection to connection.
- Check that the new bathroom sink supply line has matching connection sizes and a similar length.
- Make sure the new line can route naturally without sharp bends or twisting.
If it works: You have a replacement line that matches the old one closely enough to install without strain.
If it doesn’t: Do not force a near match. Take the old line with you and get one with the correct length and connection style.
Stop if:- The new line is clearly too short.
- The fittings do not match the valve or faucet threads.
- The old line appears hard-piped or part of a larger damaged assembly.
Step 3: Disconnect the old supply line
- Use the flashlight to identify the lower connection at the shutoff valve and the upper connection at the faucet shank.
- Hold the valve steady if needed and loosen the lower nut with the adjustable wrench.
- Let the remaining water drain into the bucket.
- Loosen the upper nut and remove the old line completely.
If it works: The old supply line is removed without damaging the valve or faucet connection.
If it doesn’t: If a nut is stuck, apply steady pressure and reposition the wrench for a better grip. Avoid excessive force that could twist the valve or faucet shank.
Stop if:- The shutoff valve starts turning in the wall or supply stub-out.
- The faucet shank spins or feels loose in the sink.
- Threads look cracked, badly corroded, or stripped.
Step 4: Install the new supply line
- Start the nuts by hand to avoid cross-threading.
- Connect the new line to the faucet and shutoff valve in the same general route as the old line.
- Make sure the line is not kinked, stretched, or rubbing a sharp edge.
- Tighten the connections snugly with the wrench after hand-threading is complete. Do not overtighten.
If it works: The new line is installed with straight, secure connections and a smooth route.
If it doesn’t: Back the nut off and start again by hand if it does not thread easily. A fitting should not need force to start correctly.
Stop if:- A nut will not thread by hand.
- The line kinks when routed into place.
- A fitting bottoms out crooked or feels stripped.
Step 5: Turn the water back on slowly
- Close the faucet if you left it open.
- Turn the shutoff valve on slowly while watching the lower connection.
- Then open the faucet briefly to purge air and repeat for the other side if needed.
- Wipe both connections dry with a towel so fresh drips are easy to spot.
If it works: Water returns normally and both connections stay dry during the first few minutes.
If it doesn’t: If you see a drip, gently snug the leaking connection a little more and dry it again before rechecking.
Stop if:- A steady leak continues after slight tightening.
- Water sprays from a fitting.
- The shutoff valve itself begins leaking around the stem or body.
Step 6: Check for leaks and finish up
- Run hot and cold water for a minute and inspect the line again with a flashlight.
- Feel around both fittings with a dry finger or tissue to catch small leaks.
- Check that the line does not rub when the faucet is used and that nothing under the sink presses against it.
- Put items back under the sink only after the area stays dry.
If it works: The sink works normally and the new supply line remains dry after use.
If it doesn’t: Check again after 15 to 30 minutes and later the same day. A slow drip means the connection still needs attention or the line may be the wrong fit.
Stop if:- You find repeated dripping after careful retightening.
- The cabinet shows hidden water damage or mold.
- You are unsure whether the leak is from the line, valve, or faucet.
Replacement Parts
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FAQ
Do I need to replace both sink supply lines at the same time?
Not always, but many homeowners replace both if they are the same age and style. If one line has failed, the other may not be far behind.
Should I use thread tape on a bathroom sink supply line?
Usually no on the supply line nuts themselves. Most sink supply line connections seal at the fitting, not on the threads. Follow the replacement line's instructions if they differ.
How tight should the new supply line be?
Start the fittings by hand, then tighten until snug with a wrench. Do not crank down hard. If it drips, a small additional turn is safer than overtightening immediately.
What if the shutoff valve leaks when I turn it back on?
The problem may be the valve rather than the supply line. If the valve leaks from the stem or body, stop and repair or replace the valve before relying on the sink again.
How do I know what length supply line to buy?
Measure the old line and choose a similar length that reaches comfortably without stretching or making a tight loop. A natural, gentle route is best.