Step-by-step fix
Step 1: Confirm the supply line is the part that failed
- Dry the shutoff valve, supply line, and nearby pipe with a towel.
- Watch closely while the water is on and look for fresh moisture forming at the line, at either compression nut, or along the tubing itself.
- Check for obvious damage like green or white corrosion, a kinked braided line, a split outer jacket, or a line that was forced into a tight bend.
- Make sure the leak is not coming from the valve stem, the valve body, or a cracked pipe just behind the connection.
If it works: You have confirmed the leak or damage is at the main water shutoff compression supply line or its end connections.
If it doesn’t: If everything stays dry, the leak may be intermittent. Place a dry paper towel under the area and recheck during normal water use to pinpoint the source before buying parts.
Stop if:- Water is coming from the valve body, valve stem, or the pipe in the wall or floor instead of the supply line.
- The shutoff assembly is badly corroded, loose in the wall, or the pipe moves when touched.
Step 2: Shut off water upstream and relieve pressure
- Locate the water shutoff that feeds this valve from upstream, such as a curb stop, meter shutoff, or another accessible main shutoff before this connection.
- Turn that upstream shutoff off fully.
- Open a nearby faucet at a lower level if possible to relieve pressure and drain as much water as you can from the line.
- Set a bucket under the work area and lay towels around the valve.
If it works: Water flow has stopped or dropped to a small residual trickle, and the line is no longer under pressure.
If it doesn’t: If water keeps flowing strongly, you have not isolated the line yet. Find the correct upstream shutoff before disconnecting anything.
Stop if:- You cannot shut water off upstream.
- The shutoff will not close and the line is still under full pressure.
Step 3: Remove the old compression supply line
- Use one wrench to hold the valve or fixed fitting steady so you do not twist the pipe.
- Use the second wrench to loosen the compression nut on one end of the supply line.
- Loosen the other end and remove the line carefully, letting any trapped water drain into the bucket.
- Inspect both connection points and wipe them clean so the sealing surfaces are free of grit, old residue, and corrosion flakes.
If it works: The old line is out, and both connection points are clean and easy to inspect.
If it doesn’t: If a nut is stuck, apply steady pressure instead of jerking the wrench. Reposition your tools so you are turning only the nut and not the pipe.
Stop if:- The valve, pipe, or fitting starts twisting with the nut.
- The threads or sealing surfaces are cracked, crushed, or too corroded to make a reliable new connection.
Step 4: Match and position the new line
- Compare the new line to the old one for length, end size, and connection style.
- Set the new line in place without forcing it. It should reach both ends naturally with a gentle curve, not a sharp bend.
- Start each nut by hand to avoid cross-threading.
- Once both nuts are hand-tight, hold the fixed fitting with one wrench and snug each nut with the other until secure. Do not over-tighten.
If it works: The new supply line is installed straight, supported by its natural shape, and both nuts are snug with no cross-threading.
If it doesn’t: If the line will not start by hand or does not reach comfortably, remove it and recheck the size and connection type before tightening further.
Stop if:- The new line is too short, has to be stretched into place, or must be sharply bent to fit.
- The nut will not thread on smoothly by hand, which usually means the fit is wrong or the threads are damaged.
Step 5: Turn the water back on slowly and check for leaks
- Close the faucet you opened for pressure relief if needed, then reopen it slightly so air can escape as the system refills.
- Turn the upstream shutoff back on slowly.
- Watch both ends of the new line as pressure returns.
- Wipe each connection dry and check again after a minute for fresh beads of water.
- If you see a slow drip at a compression nut, tighten that nut a little more while holding the opposing fitting steady.
If it works: The line stays dry as the water comes back on and the nearby faucet runs normally without sputtering after the air clears.
If it doesn’t: If a connection still drips after a small additional tightening, shut the water back off and inspect the fit, alignment, and sealing surfaces again.
Stop if:- A steady leak continues after careful snugging.
- The valve or pipe begins leaking from a different spot once pressure is restored.
Step 6: Verify the repair holds during normal use
- Run water at a nearby sink, tub, or hose bib to put the line under normal household pressure and flow.
- Check the new line and both connections again with a dry finger or paper towel.
- Look back after 15 to 30 minutes for any slow seepage that did not show up right away.
- Clean up the area but leave it easy to inspect for the rest of the day.
If it works: The new main water shutoff compression supply line stays dry during real use and after sitting under pressure.
If it doesn’t: If you find even a small recurring drip, shut the water off again and correct the connection before leaving the area unattended.
Stop if:- You see hidden moisture spreading into walls, flooring, or framing.
- The leak source changes and appears to be the shutoff valve itself rather than the new line.
Replacement Parts
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FAQ
Can I just tighten the old compression nut instead of replacing the line?
Sometimes a slight drip stops with a small tightening, but if the line is corroded, kinked, split, or keeps leaking after that, replacement is the better fix.
Do I need thread tape on a compression supply line?
Usually no. Compression-style connections seal at the fitting surfaces, not on the threads. Adding tape can make tightening feel misleading and does not fix a bad fit.
How do I know I bought the right replacement line?
Match the length, both end sizes, and the same connection style. The new line should thread on by hand and reach both fittings without being stretched or sharply bent.
What if the leak is actually from the shutoff valve body?
A new supply line will not fix that. If water is coming from the valve stem, body, or the pipe attached to it, the repair path is different and may require valve replacement.
Is this a DIY job or should I call a plumber?
It is a reasonable DIY repair if you can shut the water off upstream and the fittings are in good shape. Call a plumber if the valve is seized, the pipe moves, corrosion is severe, or you cannot stop the water fully.