"Jim Macklin" wrote in message
news:Hs3%f.3012$8q.1738@dukeread08...
Improper vent installation.

Good chance that is the case.

The vent is supposed to be in a vertical run of pipe, not in the horizontal,
and it is allowing water to have to pull the full cross sectional area of
pipe, without air replacing it, and water flowing in the bottom.
--
Jim in NC