Ender Bucket always wants to fill rather than empty itself. With Extra Utilities 2 drums(and probably others), when I use it on a filled drum the bucket always takes fluid until it's full. If I sneak and right-click, the liquid is placed in front of the drum, rather than in it, which means I can not put liquid into drums unless they're completely empty.
I wanted to use it to pick up lava, but without a good way to unload I don't see a way.
Therefore, suggestion: make sneak put liquid into a drum(or tank, or other container) if looking at one.