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ticked- look up closed loop plumbing systems. If you have your drains plumbed the way I think you do, then the pressure gradient can be different coming from each drain. I would think something as simple as an air bubble could affect it... or maybe the gravitational pull of the moon, who knows?
I know that it seems like a last resort... I wouldn't want to redo plumbing if I were you either... but it's worth looking into.
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