Sump pump and toilet problem

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.
Charles
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Charles wrote:

Not a sump pump but a sewage pump or ejector.
The pump was incorrectly installed or else there is a venting problem with the toilet on level 2.
Without seeing it, my money is on an improper install. Get a knowledgeable local contractor to take a peek...
Jim
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<< there is a venting problem with the toilet on level 2 >>
The venting problem may be more than just one level. A bad install could haunt all the tenants for years. Sounds to me like a find it and fix it project is badly needed. HTH
Joe
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