How to Replace a Bathroom Sink Supply Line

Step 1: Confirm the supply line is the problem

1. Open the cabinet and dry the shutoff valve, supply line, and faucet connections with a towel.
2. Turn the sink on for a moment, then watch where fresh water appears.
3. Look for drips from the crimped hose, corrosion at the nuts, or seepage right where the line connects to the valve or faucet.
4. Make sure the leak is not coming from the drain, the faucet body, or the shutoff valve itself.

If it works: You have confirmed the bathroom sink supply line or its end connection is the source of the leak.

If it doesn’t: If water is coming from the shutoff valve body, faucet, or drain parts instead, fix that part first because replacing the supply line alone will not solve it.

Step 2: Shut off the water and set up the work area

1. Close the sink shutoff valve for the leaking line by turning it clockwise until it stops gently.
2. Turn on the faucet to relieve pressure and confirm the water is off.
3. Place a bucket or bowl under the valve and lay a towel in the cabinet to catch drips.
4. Use your flashlight to note how the old line is routed so the new one can follow a similar path without twisting.

Step 3: Remove the old supply line

1. Hold the shutoff valve steady with one hand or a second wrench if needed so you do not twist the valve or pipe in the wall.
2. Loosen the nut at the shutoff valve end and let the remaining water drain into the bucket.
3. Loosen the nut at the faucet end and remove the old line completely.
4. Take the old line with you as a reference and compare its length, end sizes, and connection style to the replacement.

Step 4: Install the new supply line

1. Thread the new line onto the faucet connection by hand first to avoid cross-threading.
2. Thread the other end onto the shutoff valve by hand.
3. Make sure the line hangs in a smooth curve and is not stretched, sharply bent, or rubbing a sharp edge.
4. Tighten both ends snugly with a wrench after hand-threading. Do not overtighten; the goal is a firm seal, not crushed fittings.

Step 5: Turn the water back on and check for leaks

1. Close the faucet if you left it open.
2. Slowly open the shutoff valve and let the line fill gradually.
3. Dry both connection points completely, then watch and feel for any fresh moisture.
4. If you see a small seep, tighten that connection slightly and check again.
5. Run both hot and cold water for a minute if you replaced both sides, or run the repaired side through normal flow and temperature changes.

If it doesn’t: If a connection still seeps after a small retightening, shut the valve back off, remove the line, and recheck the fit and thread alignment.

Step 6: Verify the repair holds in real use

1. Wipe the cabinet floor and fittings dry one last time.
2. Use the sink several times over the next few hours, including a normal hand-washing cycle with warm and cold water.
3. Check under the sink again for slow drips, dampness on the line, or water collecting around either nut.
4. Leave a dry paper towel under the connection area for a final check if you want an easy way to spot a slow leak.

If it works: The bathroom sink supply line stays dry during normal use, so the repair is complete.

If it doesn’t: If you find new moisture later, trace it carefully to confirm whether the supply line connection needs adjustment or another nearby part is leaking.