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The lack of resistance to the incoming force of water makes all the difference. Without back pressure, all that is happening by filling the vent pipe is adding another 4+ lbs of weight to a tank already holding 400+ lbs of static weight.
BTW, I've had plumbing tested in the example you gave. It pressurizes drain lines to a low pressure to test for leaks in a drain system not designed to be pressurized at all. That's how 4 lbs of pressure is all that's needed.
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2016 3160, Legacy, Sailuns, Splendide 2100 xc vented, 1 1/2" axle lift blocks, disk brakes. 2014 Ram 3500 SRW SWB 4X4 6.7 Aisin Mega Cab, EBC slotted disks and brakes, Titan fuel tank.
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