Toilet flapper — pencil-mark the tank waterline after a quiet fill
Running toilet
After the tank goes quiet, pencil-mark the waterline so you can see if a slow toilet flapper leak drops the level — one small beat.
When a toilet flapper weeps slowly, the tank level drops before the fill valve kicks on. After a quiet fill, pencil-mark the waterline on the tank wall — a drop below the mark later confirms a leak without guessing.
Pencil-mark the toilet tank waterline after a quiet fill
- Flush once, wait until the fill valve stops and the tank is silent, then open the lid and dry a small spot on the tank wall at the water surface.
- Make a short pencil mark at that waterline (graphite only — nothing that sheds into the tank). Close the lid and leave the toilet unused for 20–30 minutes if you can.
- Reopen and compare: if the water sits clearly below the mark with no one flushing, the flapper (or a related seat) is leaking; if the mark still matches, look at the fill valve or handle next.
- When you confirm a drop, hold the flapper closed by hand as a quick second check, then replace or reseat the flapper — erase and remark after the repair.
Honest stop: do not use permanent marker, paint, or anything that can flake into the tank water — pencil only, and keep marks off the flapper rubber.
More help
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