wrote:
Hello,

I have an older toilet. A few years ago, I
changed all the parts inside the tank. It
has worked fine since then. Until now.

Lately, it has an odd pattern. I flush the
toilet. The tank and bowl refill fine.
I've watched them.

Then I come back next time. And the bowl is
almost empty!

And, it only seems to happen intermittently.

It's really bizarre.

Does anyone have any idea what might be
causing this, and the solution?

Thanks a lot!



from
http://www.askthebuilder.com/B332_To... lems_.shtml:

You flush the toilet and all is well. After a period of time, a
significant amount of water has left the bowl. Two things may be wrong.
Water could be slowly siphoned from the bowl by a partial clog of toilet
paper up in the colon of the bowl. You can demonstrate this phenomenon
by filling a small soup bowl with water and putting it in the center of
a cooking jelly pan. Drape a strip of paper towel from the bottom of the
bowl, over the bowl edge and into the jelly pan. Watch what gravity and
capillary attraction does in several hours. The bowl will be nearly
empty. To see if your toilet has a rag, toilet paper, or something else
causing the drainage, empty the bowl of water and then use a flashlight
and a mirror to look up inside the colon of the toilet.

In rare cases, the bowl may actually have a crack in the interior colon
or piping of the bowl. This problem can only be solved by installing a
new bowl.