Water Disappears When Transporting/Flowing

Pumping water from one place to another causes rapid losses in volume, particularly with a vent.

Emptying out the whole top area into the bottom area yielded ~2 blocks deep of water, big loss there...

Then pumping it down into the tank next to my purifiers caused a complete loss in any visual water, however, 

It says 4000g of water, fluctuating between that and ~17 to ~70 g. NOT Kg. Is this a problem of units, or is it the use of the vent that's causing this, producing much less water than it actually has in the pipes?

EDIT: Sorry in that image the pump has no power, but even when powered the pump will not pump, or it does for a tiny amount and then stops.