In a three-story house built on the side of a hill, the bottom floor
(level 3) has an apartment equipped with a sump pump to pump waste water
up to sewer level.
When the sump pump goes off, water in a toilet bowl upstairs on level 2
is first sucked down a little, then rebounds with a splash. With each
successive pumping the water level in the bowl gets a little lower.
Any suggestions very much appreciated.