Just got my Big Berkey water filters, and made my own 5 gallon water bucket system. Seemed to work good, passed the red color test. The only problem was when it got down to about 1 gallon of water to filter. It seemed extremely slow, almost non existent water dripping. I seem to remember reading that the black water filters require a lot of water to get the necessary pressure to filter water. Is this normal with the berkey water filter? Slow or non existent dripping with about 1 gallon of water in the top chamber?