I have a two story house, with two bathrooms up and a half bathroom on the main level. My shower in the master bathroom will smell like sewer in mid-shower ( I don't think that it is me either ha, ha). Also, when we flush the master bathroom toilet, the other toilet upstairs will run for a half second, but not all the time.

Do you think that the main stack could be plugged?

It sure sounds like it -- And it would explain why the p-trap in the shower is being suctioned out while in use.

And that is why the second toilet draws down also.

How do you prim an above ground water pump?