Andrew Gabriel wrote:
In article ,
(D.M. Procida) writes:
Our downstairs toilet seems to half-empty itself after flushing. I am
sure it never did this before. You can flush it, it seems normal, and
half an hour later the level in the bowl has dropped by a few
centimetres.

What could be going on? There's nowhere for it to go other than escaping
by climbing over the U-bend, and no sign of any leak. I am pretty sure
that no-one in the household is siphoning it away or drinking it.

A change in behaviour (if it didn't always do this) can
be caused by a blocked sewer. Levels changing in the
sewer can suck U-traps whereas they would normally be
better vented when the system isn't full of you-know-what.

--
Andrew Gabriel

There are three possibilities I know of: a leak, trap sucking and wind.

A hairline crack in the ceramic below the water level might well be
hard to spot, especially if it's round the back and the floorcovering
allows the water to disappear under the fitting rather than appearing
in front. Moving the floor covering and getting right down there with
a mirror if neccessary is unpleasant but could be worth it.

Andrew's description of a blocked sewer shouldn't be able to cause the
effect as there is supposed to be either an Automatic Air Vent or an
open pipe to allow plenty of air into the sewer. Now in the real world
things don't always go to plan and Andrew is of course correct that a
blockage could be sucking out the trap.

If it's particularly windy where you are you might find that air
flowing over the top of the open vent pipe can cause this - sort of a
special case of the trap being sucked out. In our house it only ever
happens if there's a gale blowing from the north.

Good luck,
Calvin