I'm wondering if there's another way to reduce pressure in the main sewer relative to your drain lines.
What if they flushed the line with a large volume of water? The sewers I've had open have usually had a small amount of the line actually full of water, just a small flow at the bottom, and your line dumps in well above that. But if they were flushing with lots of water, the moving water might rise above your inlet, and pull your water out with it.
Not sure this happens but seems like it might be possible.