it could be. other times the under-the-sink view includes an old style
S trap which seems to make more resistance to the water to drain than
the P traps of today.
i have an oversized 2" S TRAP under my 1-1/2" kitchen sink with
strainer basket removed and am seeing the backup problem unsolved by an
add-a-vent under the sink even when replaced by a new one.
i see any temporary backup water miraculously disappear when dish
detergent is squirted into the sluggish drain.
[i am wondering if the 2" s trap must be a 1-1'2" p trap; [any more
help would be welcome on this]
see your rooftop for location of actual 4" soil stack pipe rising above
toilet rooms and sometimes separate venting for a new bath or kitchen
thru the roof in a 2" pipe.
SEE VENTING AT:
http://www.free-ed.net/sweethaven/Bl...p?iNum=fra0309
it could be blockage after the toilet and after the sink but before the
soil stack.
see:
http://www.fluidmaster.com/
SEE S-TRAP PICTURES AT:
http://images.google.com/images?sour...an&q=s%20traps
SEE P TRAPS AT:
http://images.google.com/images?svnu...ps&btnG=Search


Eigenvector wrote:
I have one drain in my bathroom that drains normally, but every so often it
won't drain until a certain depth is in the basin (sink) then drains keeping
that level in the basin. After it drains, or when the water flow slackens a
bubble will come out of the drain and then the water will all drain from the
basin. It's not repeatable, but I wonder if it might also be responsible
for my occasionally slow to drain toilet.

Is this what a venting problem does, or am I describing something different?