Quick Answer:
When sediment or debris moves through a line it can lodge on a valve seat and prevent a tight shutoff. Flush the line first to clear loose material, then only snug the packing nut/bonnet slightly to see if the leak stops. If the valve still won’t seal after flushing and gentle tightening, rebuild or replace the affected valve. Avoid repeatedly opening and closing the valve until debris is cleared — that can make the problem worse.
Why This Happens
Water systems, especially after work on the main or when a tank discharges solids, can push grit and flakes downstream. Those particles catch on the smooth, small contact area of a valve seat or on the disc/screw. Even a thin film of debris prevents metal or rubber sealing surfaces from mating tightly. Over time seats can be scratched, and seals fail more easily.
If you use a water heater or flush a tank, you can move sediment through the plumbing; see Water heater fills with debris for more about how heaters can introduce solids. Sediment can also shorten the life of meters and mixers — read about Sediment damaging appliances for details on related risks.
Step-by-Step What to Do
Step 1 — Shut off and flush the line
- Close the nearest upstream supply or the main shutoff to stop flow into the problem branch.
- Open a downstream fixture or drain to remove water and any loose sediment. Flushing the line first is essential — it often clears the debris and restores the seal without further work.
- Run water long enough to see clear flow; brown or gritty water means keep flushing.
Step 2 — Test carefully; avoid repeated cycling
- After flushing, briefly open and close the valve to test if the seat now seals. Do not cycle the valve repeatedly — that can move more debris over the seat or damage soft seals.
- If the valve only weeps, a gentle test is all you need before trying to tighten the packing.
Step 3 — Snug packing nuts/bonnets only slightly
- Use an adjustable wrench to turn the packing nut or bonnet in small increments (1/8 to 1/4 turn). Tighten only enough to stop a small weep. Over-tightening can damage the stem or packing and create a worse leak.
- Wait a few minutes after each small adjustment to see if the leak stops before turning more.
Step 4 — If the valve still won’t seal, rebuild or replace it
- If flushing and gentle snugging don’t work, remove and inspect the valve internals. Seat and disc surfaces may need cleaning, resurfacing, or replacement.
- Use a rebuild kit or replace the valve with the same type and size. If you find heavy scoring or pitting on the seat, replacement is the safest choice.
- When replacing, take care to re-flush the line before completing installation so fresh debris won’t damage the new valve.
Step 5 — Restore service and monitor
- Open supplies slowly to avoid a sudden surge of any remaining debris.
- Confirm there are no leaks and check again after a day of normal use to ensure the seal holds.
What Not to Do
- Don’t over-tighten leaking valves. Forcing the packing or seat can strip threads, damage stems, and create larger leaks.
- Debris on the seat is often the cause—flush before rebuilding. Jumping straight to aggressive repairs without flushing usually wastes time and parts.
- Don’t use excessive force to free a stuck valve; you may break the valve and create a worse failure.
When to Call a Professional
Call a plumber if any of the following apply:
- Valves are on a gas line or near gas piping (do not work on gas valves yourself).
- Multiple valves or fixtures leak after flushing — this may indicate a larger supply issue.
- You find heavily corroded or broken seats, or the valve requires soldering or pipe rework you’re not qualified to do.
- The valve controls critical equipment (boilers, main shutoffs) and you prefer a professional replacement.
Safety Notes
- Turn off the correct upstream supply before disassembling a valve; relieve pressure by opening a downstream tap.
- Expect hot water when working near hot-water lines — allow cooling time or shut off the water heater if needed.
- Wear eye protection and gloves to avoid contact with dirty water and sharp metal fragments.
- Never use an open flame on plumbing near plastics or gas lines. If soldering is required, shut off and vent gas first and follow local codes.
Common Homeowner Questions
- Why did this start suddenly? A change in flow (main work, tank flush, or debris release) often moves sediment that then lodges on a valve seat.
- Can I just tighten the valve more to stop it? No — over-tightening often worsens the damage. Snug the packing slightly, but flush first and consider rebuild or replacement if it still leaks.
- How long until the line clears after flushing? Usually a few minutes of steady flow; if water stays discolored or gritty, continue flushing and consider isolating the source of sediment.