This week I'm staying with a friend at her Ocean City beach house. It was closed up for a while and she turned the water on for the first time in a few months, The toilet in "my" bathroom filled but they the water came on again. Leaking flapper, but it is not a flapper style.
I looked at it and there is a cylinder that moves up when you hit the handle. No idea how to check it out, so went to YouTube and found the instructions.
Once I flipped the cylinder over, I saw the split that cause lots of water loss.