I have a sump pump connected to a pvc line that goes out to city line. I dump my washer water into the same line and also have a sink that is connected with a y fitting to the same line. When the pump or Washer is pumping the water comes back up through the sink even though I have a y fitting. How does the water come back up the pipe? Does it have something to do with the main venting pipe that goes up through my roof? Don’t understand how the water comes back to the sink.

So let me be clear, your city allows your sump pump pit be connected to city sewer, right? How large is the PVC line, its either partially clogged or the pump can simply put more water out there than the line can handle. The 2009 plumbing code will start requiring washer drain to be 3" because of more powerful pumps.