- 1). Locate the water line inside the tank. This line is often marked on the overflow tube, which is the vertical pipe near the center of the tank that is a part of the flush valve assembly. If the water line is not marked on the overflow tube, find it on the inside back wall of the tank.
- 2). Measure from the top of the overflow tube, down about three-quarters of an inch, using the tape measure. This is the proper water level for most tanks. The water level should not reach the top of this tube.
- 3). Close the shut-off valve on the supply line running from the toilet completely counterclockwise. Allow the water to fill the tank until it shuts off. Compare the resulting water level to the water level mark on the inside tank wall or the overflow tube. Verify that the flapper seats properly and that no water is leaking from the tank through the flush valve seat.
- 4). Adjust the fill valve, according to the specific toilet manufacturer's instructions, to raise or lower the water level to line up with the water lines. Flush the toilet and allow the tank to finish filling with water. Continue adjusting the fill valve, as needed.
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