I found this out the hard way the other day when I was moving one of the valves on my oil tank, and lost half the oil by doing it the wrong way... If you want to move a valve to a different spot in the tank, break the valve first. If you break a non-valve block, then break the valve while the tank is unformed, the liquid that valve is holding will disappear. If you break one of the valves in a tank while the tank is still formed, the other valves will keep all of the liquid, so you don't lose any when you reform it.