I recently replaced both my toilets' fill valves and flappers to resolve some running water/leaking issues - which was fixed on both toilets. The toilets are Kohler 1.6 gpf models.

But now my problem is that the bowl water fill level is about an inch lower than it used to be.
As soon as the bowl is finished filling (which seems to be at a normal level initially) - the water level immediately drops about 1-1.5 inches and then stays there.

Here's the things I've tried from other research without much success:

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I don't see any external leaks around the toilet.

I’ve checked that the flapper chain has enough play for the flapper.

I've checked that the fill valve hose is positioned correctly.

I've raised the fill valve floater to as high is it can be while maintaining the water line to where it should be.

I've tried "slowing" down the water fill by turning down water pressure. Some say a tank that fills to fast won't allow the bowl to fill to normal capacity.

I've checked the little holes along the toilet rim and they seem to not be clogged/blocked.

I've forced the trap to clear by turning off the water while pouring multiple gallons of water down the toilet which flushes with good amount of force - so
There doesn't seem to be any clogs there.

I have not checked the vent on the roof to see if it blocked which would might cause siphoning in the toilets - but I don't think this is it because if it was - wouldn't the water level in the toilet bowls drop much more? The drop in level I see is relatively small - but still not as high as it should be.
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I used the Fluidmaster 400A fill valve kit and
Flappers as the replacements on both Kohler 1.6 gpf toilets.

I'm running out of ideas, any suggestions on what else to check or what could be the cause?

Thanks

Some say a tank that fills to fast won't allow the bowl to fill to normal capacity. Common sense will tell you that's wrong!

I've checked that the fill valve hose is positioned correctly.
But did you check the stream? Disconnect the refill tube, (see image) from t6he overflow tube and flush the toilet. There should be a strong stream out of the refill tube as the tank fills. If not shut the water off, open the ballcock and see if the ports clear. Let me know, Tom

Check your refill tube and make sure it is installed according to the specs.

I checked the refill tube and it has a good stream coming from the fill valve.

I've noticed now that if I leave the toilet overnight without flushing, that the water level drops considerably more - to the point where there is just enough water to cover the trap.

From other posts I've read, it sounds like this could be a blocked vent on the roof. That or maybe a crack in the toilet somewhere?

I've checked the toilet externally and didn't see any small leaks or anything. I hope its not a broken toilet because this toilet isn't that old.

Thanks

I've noticed now that if I leave the toilet overnight without flushing, that the water level drops considerably more - to the point where there is just enough water to cover the trap.
Unless you see the water bouncing in the bowl when you drain a tub I don't think you have a vent problem. Sounds like the bowl's leaking back down into the sewer. This needn't be anything that you've done to the bowl, read on.
There could also be a factory defect in the bowl. Sometimes in casting the bowl there is a small bubble in the china. Instead of junking the bowl the manufacture will put a porcelain patch over the hole and send it on through. On occasion the patch will fail allowing the water to seep out of the bowl. If that's the case you would have a full bowl after you flushed and the bowl filled but some time later you would notice a drop in the bowl level. The only other thing might be if you dropped something in the bowl and cracked it. If you're worried about where the water's going, in both cases it drains back down into the sewer and not under the bowl on the floor. To check the bowl must be pulled, filled and put up on saw horses. You can then see if there's any leakage from the bowl. Good luck, Tom

Thanks for the information.

I left the other toilet overnight without flushing, and its level did not drop significantly like the other one. Just a an inch or so right after flushing - which both toilets do since replacing the fill valve and flappers on both.
I'm wondering if there is something wrong with the fill valve and/or flapper, because I did not see this drop in water level until after I replaced the fill valve and flapper.

Someone else mentioned that the water level may be going back down to where it should be, since I replaced the broken fill valve and flapper. Before the toilet would fill every so often and maybe that is why the water level was always higher before I replaced the fill valve and flapper, and now I'm seeing where the water level should be. Is there a way to check for a factory defect in the bowl without removing it?

Thanks

Is there a way to check for a factory defect in the bowl without removing it? To check the bowl must be pulled, filled and put up on saw horses. You can then see if there's any leakage from the bowl.

Before the toilet would fill every so often and maybe that is why the water level was always higher before I replaced the fill valve and flapper, and now I'm seeing where the water level should be.
That could be one explanation. You're sure the tank levels are a half to three quarters of a inch below the lip of the white overflow tube? Tom