I'm not sure about the meaning of a test I did. First, let me explain that
my grey/rain water
all empties into a gully on the side of my house. I think it empties into a
salt-glazed sewer pipe.
The sewer pipe is buried under a concrete ledge running along the side of my
house.

After about 8 feet horizontally a soil pipe from the upstairs loo enters the
same concrete ledge, and joins the same sewer pipe, i.e. my sewage and
rainwater all empty into the same pipe. Just before the sewer pipe reaches
the road there is an inspection chamber, and whilst looking in there for a
completely separate reason, I rashly decided to do a leak test.

I bunged up the sewer pipe in the inspection pit, then went to the gully and
ran water into it until it
filled up the ceramic pipe draining the gully to the lip. The water level
then dropped two inches in some 40 seconds or so, then dropped much more
slowly. I repeated the test and listened for what I could hear along the
route of the sewer pipe, and distinctly heard water gurging for 40 seconds
where the soil pipe enters the concrete ledge.

Sorry to bang on in such detail, but my question is, would you folks expect
the ( unseen ) join between a soil pipe and a sewer pipe to be airtight, or
is it arranged so that flowing water coming down the soil pipe negotiates
the join ok, but standing water being pushed back up a few inches
above that join will leak out?

Do I have a problem, or am I meddling with stuff I don't understand? :-)

Andy.