Toilet flushes and refills normally, but water very slowly drains over time after; also bobbing?

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94000130 FFF70000

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I know basically nothing about toilets or plumbing, nor does anyone else I know. I had to replace the plug thingy in the bottom of the tank on the back once and I know that's important to work right and I don't really understand why. I don't know how to check for leaks besides "is there water on the floor?".

My problem is exactly what the title says. The toilet flushes and refills normally, but water very slowly drains over time afterwards (after the point where it's definitely not supposed to be draining anymore).

The water also bobs a bit during this process, which is surprisingly noticeable; I don't remember ever seeing this happening with this toilet before in cases wherein the water was slowly going down (due to clogs) so I took a video so anyone who recognizes it can narrow down the problem further.

https://drive.google.com/file/d/1Pp63raP2kqmb-BvGuDg3TpZGX3PCg7yp/view?usp=sharing

Why would this be happening? I don't see a leak anywhere, nor any water on the bathroom floor. My folks think it must be fine to leave it - there's no phantom flushing or anything - but I'm not so sure, and I'd really rather get a problem dealt with sooner rather than later, especially if later involves a bigger mess. I've seen posts noting the possibility of hairline cracks, but I'm not sure if this behavior is typical for one of those, nor if there could be another explanation.

I couldn't figure out how to search for this specific problem in a way that didn't fill all the results with more standard your-toilet-is-obviously-partially-clogged advice, so sorry if there's already a thorough explanation for what's going on here that I missed.
 

Terry

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Sometimes something like dental floss in the toilet bowl trapway can wick water out of the bowl.
Sometimes a plugged main line can cause water to siphon out of the bowl.
A back to back toilet can push a bit of water out if flushed.
 

Tuttles Revenge

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The water movement is likely caused by the flushing of another toilet in the system and its located directly behind the problem toilet or some venting issue is causing pressure against the toilet trap arm.

In order to understand why some water disappears from the bowl when the water moves, fill up a bowl with water to the very top. Then attempt to walk up stairs with that bowl of water. Likely some water falls over the edge. In a toilet we call that the Weir. Its the point that water begins to fall down the drain. Any water that goes over the weir disappears. The toilet bowl always fills up to the weir of the bowl. So if your bowl water sloshes around at all, some of that water will fall over the weir.

Your location is asked for so that people can give location specific advice in cases where plumbing code questions are asked.
 

94000130 FFF70000

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The water movement is likely caused by the flushing of another toilet in the system and its located directly behind the problem toilet or some venting issue is causing pressure against the toilet trap arm.

In order to understand why some water disappears from the bowl when the water moves, fill up a bowl with water to the very top. Then attempt to walk up stairs with that bowl of water. Likely some water falls over the edge. In a toilet we call that the Weir. Its the point that water begins to fall down the drain. Any water that goes over the weir disappears. The toilet bowl always fills up to the weir of the bowl. So if your bowl water sloshes around at all, some of that water will fall over the weir.

Your location is asked for so that people can give location specific advice in cases where plumbing code questions are asked.

So something is moving the water and that's causing the water level to go down, then, thank you. It makes sense when you put it like that, and matches what I've been able to observe since the original post (the severity of the bobbing varies, and the water level goes down more at the same time that the bobbing is more severe).

It's not another toilet flushing, there's no other toilet on this floor and the others weren't in use (I checked), and I'm positive nothing unusual has been flushed lately, so that leaves... a venting issue, most likely?
 

Tuttles Revenge

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Could be something in the way its vented or not.. it could be that the main sewer has a belly or is backed up to some degree not allowing air to pass through the system fully.
 

AquaPlummer

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I know basically nothing about toilets or plumbing, nor does anyone else I know. I had to replace the plug thingy in the bottom of the tank on the back once and I know that's important to work right and I don't really understand why. I don't know how to check for leaks besides "is there water on the floor?".

My problem is exactly what the title says. The toilet flushes and refills normally, but water very slowly drains over time afterwards (after the point where it's definitely not supposed to be draining anymore).

The water also bobs a bit during this process, which is surprisingly noticeable; I don't remember ever seeing this happening with this toilet before in cases wherein the water was slowly going down (due to clogs) so I took a video so anyone who recognizes it can narrow down the problem further.

https://drive.google.com/file/d/1Pp63raP2kqmb-BvGuDg3TpZGX3PCg7yp/view?usp=sharing

Why would this be happening? I don't see a leak anywhere, nor any water on the bathroom floor. My folks think it must be fine to leave it - there's no phantom flushing or anything - but I'm not so sure, and I'd really rather get a problem dealt with sooner rather than later, especially if later involves a bigger mess. I've seen posts noting the possibility of hairline cracks, but I'm not sure if this behavior is typical for one of those, nor if there could be another explanation.

I couldn't figure out how to search for this specific problem in a way that didn't fill all the results with more standard your-toilet-is-obviously-partially-clogged advice, so sorry if there's already a thorough explanation for what's going on here that I missed.
I had a similar issue at a fire station. It was a wall mount spud-type toilet. The toilet flushed great, as did the adjacent toilet, urinals, etc. After the toilet was flushed and filled up the water level would slowly drop, just to the point where sewer gas would get through. I started troubleshooting and checked the usual things, like the ones in the other replies- venting, stoppage/partial stoppage, a cracked bowl, and on and on. Long story short- When I finally pulled the toilet I looked inside the outlet and found a 1/4" hole at the bottom, just inside the horn. The holes are there to aid in the cooling of the china when manufactured. The holes are then sealed. Anyway, this hole and the siphon jet share a common waterway, so this was where the water went. No apparent leaks or anything. After 23 years this was definitely a first. Pretty strange!
 
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