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When I flush my toilet, the water level will drop about an inch after the tank fills and then fill up again. This goes on non-stop UNTIL I turn the supply off. If I turn the supply off immediately after the tank fills, then the water level does NOT drop. I have replaced the flapper, adjusted the flapper, and disassembled the fill valve in the tank to check for calcium deposits. I have cleaned the seal under the flapper. I have put petroleum jelly on the flapper. It does not make sense that the leak does not happen if I turn the supply off immediately after the tank fills. Any ideas as to what is going on and how to fix it?

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  • 1
    Can you confirm that the maximum fill level is the same with supply valve open and closed?
    – crip659
    Jun 18 '21 at 13:21
  • I don't see a difference with the supply valve on or off.
    – Lane
    Jun 18 '21 at 15:28
  • Was kind of hoping that it was over filling and draining out the over flow pipe. From what you mention, it sounds like inlet pipe is sucking water out, but that is impossible(I hope). This problem usually caused by a leaking flapper, but then supply valve has no effect on stopping the leaking.
    – crip659
    Jun 18 '21 at 15:52
  • When you flush, do you hold the lever down till tank almost empty or let go right away? Try and do it different and see if any difference.
    – crip659
    Jun 18 '21 at 16:37
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This sounds like a case with the bowl fill line that refills the bowl while the tank is refilling is the problem.

What may be happening is this line is low enough in the tube and when the float stops the incoming water flow it starts a siphon from the tank and once low enough it will refill the tank and start the cycle again.

The fix is cutting the tube off at the top of the pipe above the water line this will usually stop the siphon.

Many new refill valves actually have a clip that holds the tubing about a ~1/2” above the tube to stop this from happening.

Try ether cutting the tubing shorter or raising it above the water line and that should stop the siphon.

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  • I tried raising the bowl refill tube. It seems to be good so far, but I'm going to wait at least another day or so before I mark this as the answer because one other time, the problem did stop and come back later.
    – Lane
    Jun 19 '21 at 12:07
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    After a few days, the problem has not returned. I'm going to mark Ed Beal as the answer. A friend had already purchased some parts for me, so I'm going to have him replace them just in case anyway. Thanks
    – Lane
    Jun 21 '21 at 11:38

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