Step-by-step fix
Step 1: Confirm the supply line is the problem
- Look behind the toilet and trace the flexible line from the shutoff valve at the wall or floor to the fill valve under the tank.
- Check for drips, corrosion, bulges, kinks, or mineral buildup on the line and at both connection nuts.
- Wipe the line and fittings dry, then flush the toilet and watch while the tank refills.
- If water appears at the line or either end connection during refill, replacing the toilet supply line is a reasonable next step.
If it works: You have confirmed the leak or restriction is at the supply line or its end connections.
If it doesn’t: If the leak is coming from the shutoff valve body, the fill valve shank, the tank bolts, or the base of the toilet, fix that part instead of replacing the line first.
Stop if:- The shutoff valve is badly corroded, loose in the wall or floor, or will not turn off.
- The fill valve shank under the tank is cracked or spinning loosely.
- You see water damage in the floor, wall, or ceiling below the toilet area.
Step 2: Shut off the water and drain the line
- Turn the toilet shutoff valve clockwise until it stops.
- Flush the toilet and hold the handle down to empty as much water from the tank as possible.
- Place a towel under the shutoff valve and set a small bucket or bowl below the supply line connection.
- Open the tank lid and confirm the tank is no longer refilling.
If it works: The water is off and the line is ready to disconnect with only a small amount of water left inside.
If it doesn’t: If the tank keeps refilling, the shutoff valve is not closing fully. You may need to shut off the home's water before continuing.
Stop if:- The shutoff valve leaks from the stem or body when you turn it.
- You cannot stop water flow to the toilet safely.
Step 3: Remove the old toilet supply line
- Use the wrench to loosen the nut at the shutoff valve first while supporting the valve if needed so it does not twist.
- Let the remaining water drain into the bucket.
- Loosen the upper nut where the line connects to the fill valve under the tank and remove the old line.
- Compare the old line to the new one for overall length and matching end connections before installing anything.
If it works: The old line is off and you have confirmed the replacement matches the connection type and length.
If it doesn’t: If the new line does not match both ends or is much too short or too long, pause and get the correct replacement before installing it.
Stop if:- The shutoff valve or fill valve threads are damaged, cracked, or badly corroded.
- A fitting will not loosen without twisting the valve or tank connection.
Step 4: Install the new supply line by hand first
- Thread the top nut onto the fill valve shank by hand until it turns smoothly and seats evenly.
- Thread the bottom nut onto the shutoff valve by hand the same way.
- Make sure the line hangs naturally without a sharp bend, twist, or side pull on either fitting.
- Tighten each connection snugly with the wrench, using small turns after hand-tight. Do not force the nuts down hard.
If it works: The new line is installed straight, untwisted, and snug at both ends.
If it doesn’t: If a nut will not thread smoothly by hand, back it off and start again to avoid cross-threading.
Stop if:- The fitting starts cross-threading or the valve body begins to twist in the wall or floor.
- The line must be bent sharply to reach, which means the length is wrong.
Step 5: Turn the water back on and check for leaks
- Slowly open the shutoff valve counterclockwise while watching both ends of the new line.
- Let the tank fill completely.
- Dry both fittings with a towel, then touch around each connection with a dry finger or tissue to catch slow drips.
- If you see a drip, tighten that connection slightly and check again.
If it works: The tank fills normally and both connections stay dry under pressure.
If it doesn’t: If a connection still drips after a small retightening, shut the water back off, disconnect that end, inspect the threads and alignment, and reinstall carefully.
Stop if:- Water sprays from a fitting or the shutoff valve begins leaking from the body or stem.
- The fill valve connection under the tank leaks from the valve itself rather than the supply line nut.
Step 6: Test the repair in real use
- Flush the toilet several times and watch the line during each refill cycle.
- Check that the line does not rub sharply against the wall, tank, or shutoff valve.
- Look again after 10 to 15 minutes for any slow seepage around the nuts or on the floor.
- Wipe the area dry one last time and leave the shutoff valve fully open if everything stays dry.
If it works: The toilet refills normally and the new supply line stays dry through repeated use.
If it doesn’t: If the toilet still fills slowly with no leaks at the new line, the restriction is likely in the shutoff valve or fill valve rather than the supply line.
Stop if:- You find recurring moisture that is actually coming from tank condensation, a leaking shutoff valve, or another toilet part.
- The floor around the toilet becomes wet and you cannot clearly trace the source.
Replacement Parts
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FAQ
Do I need thread tape on a toilet supply line?
Usually no. Most toilet supply lines seal with built-in washers or cone seals at the connection points, not with thread tape. Adding tape where it is not meant to be used can actually make sealing worse.
What size toilet supply line do I need?
Match both ends and the length of the old line. Many toilets use a 7/8-inch ballcock connection at the tank and a 3/8-inch compression connection at the shutoff valve, but you should verify your toilet before buying.
How tight should the new supply line be?
Hand-tight first, then a little more with a wrench until snug. If it leaks, tighten in small increments. Overtightening can damage the fitting or washer.
Can I reuse the old supply line?
It is better to replace it once it has been removed, especially if it is older, kinked, corroded, or leaking. A new line is inexpensive compared with water damage from a repeat leak.
What if the toilet still fills slowly after I replace the line?
The restriction may be in the shutoff valve or the toilet fill valve instead of the supply line. If the new line stays dry and the flow is still weak, inspect those parts next.