It sounds more like a bad bowl.
If you pour water slowly into the bowl, it should find it's own level.
Unless there is a plug in the trapway that prevents free movement of water.
I've researched this and think I've found the answer, but wanted to see what you thought.
I have a fairly-new Kohler toilet on the top floor of my house. On the bottom level, I have a new Toto Drake. When it gets windy, the water level in the bowl of the Kohler goes down to almost nothing. The Toto is fine.
When the weather is not windy, the water level stays the same in the Kohler. The bowl level has always been less than optimal, but sufficient. When it flushes, it fills the bowl to an optimal level, then gradually sinks down a couple of inches.
Is it possible that the toilet vent is "too" open, causing the bowl to be siphoned by the wind?
I'll try pouring water into the bowl. I'll also try the food coloring trick that I saw elsewhere.
After more research it seems that if it was a bad vent, I would hear gurgling sounds from the drain or smell sewer gases. None of this is present. Also, when I flush the downstairs toilet and sink, it doesn't have any effect on the bowl level in the upstairs.
Will post results, if any! Thanks.
Just a note on water level in the bowl. In a normal bowl, water level is determined by the trap built into the toilet. There may be other factors causing the problem.
Question, did this toilet ever work right reguardless of the wind since it was installed or has it been this way since day 1.
So it seems I have a super-short vent pipe on the roof. When the wind blows, it blows right down the pipe and I can see the water in my upstairs toilet bowl moving up and down.
This toilet has always been on the low side since I moved in. Thanks to everyone for your advice.