Strange behavior with the toilet

I don't know what you mean by "older toilet" but if it's anything like the conventional home toilet, you obviously have a leaking rubber stopper. The refill valve/float assembly is also very likely in need of replacement since it does not respond to the drop in water level as the water seeps out.

You can get all this stuff for under $10 probably and install it yourself.

One reason could be high winds effecting the vent pipe which causes waves to lower the level.

Or could be a blocked outlet that given time will lower its level but in doing so will influence the bowl by a siphoning action.

Make sure there are no obstructions in your vent stack, dead squirrel, blob of leaves etc. Other than physical damage to the bowl the only thing removing water besides a flush would be a vent problem or dogs drinking. If the vent is clogged then a suction can be applied to the bowl by other parts of the plumbing/sewer system. Sometimes in very high winds you can see the water in the bowl move as the wind passing over the stack changes pressure inside.

Or a blocked vent that causes a siphoning action.

He said BOWL, fella, NOT tank.....(Do you know the difference?)

Because you say it's intermittant, I'm skeptical about others' suggestions that it's a wet strip of "something" syphoning the bowl, or a crack bypassing the trap.

I'd think more likely it's a clogged vent stack and draining something elsewhere in the house is "sucking" the water out of that toilet bowl.

HTH,

Jeff

Sheesh fella, speaking of bowl, who pissed in your corn flakes this morning? Believe it or not, I am not perfect.

So, I'm 1.778*10^10 angstroms tall. Looky, I have a calculator too!:)

Sounds like you have a clogged vent stack somewhere in the house. When something else in the house drains (eg, a washing machine), it acts as a venturi pump. Normally it would pull air down the vent stack into the drain system but if that is blocked instead it pulls the standing water out of your toilet.

Una

You stuffed up the replacement of the parts.

Get someone else to do a better job, or work out what you stuffed up.

Looks like you managed to stuff up two different areas, the main stopper clearly allows the water to leak away, and for some reason the valve that is supposed to refill it when that happened isnt working properly either.

I had this happen but the vent stack wasn't clogged, rather the vent stack was being used as a roof drain. During heavy rains so much water went down the vent stack through the poop pipe that it would suck the water out of all toilets in the building. I fixed this by raising the vent stack so that rain water would drain through it any more.

I'd question your dog.

We have the same problem here... Mine is caused by a crack in teh bowl that allows water to seep through to teh drain (but not the floor thank God!) and go out slowly over the course of 4-6 hours... Sometimes it disappears but whenever we do a good cleaning, it reappears. We presume it is the "stuff" that either gets stuck in the crack or grows there...

My neighbor had the same problem. theirs turned out to be a small piece of dental floss caught in the bowl drain that would not come loose (???) and acted like a candle wick allowing water to drip down the drain all day...

I'm guessing that yours might be caused by item #1 above and that you shoudl pull the bowl to look for water leaking into the floor below the bowl ASAP lest you have rotting problems.

I have a unvented basement toliet here that started doing this, a week or so later found main drain clogged.

my theory, up till total clog things would back up then flow violently sucking water out of bowl......

I put rocksalt in washtub by next day things were moving again

Do you have a dog?

Ok, here is an update. I closed the intake valve and went away for a few days.

When I came back the water in the bowl was down once again. But, the water in the tank had not moved.

Here's another piece of info.I live in a townhouse with a flat roof, and the roofers are replacing the roof at the moment.

Does it make sense that it would be vent pressure sucking the water out?

Thanks a lot!

Yes, wind across the vent could do that.