From http://mod-buildcraft.com/forums/index.php?topic=593.0

On page above people discussed about Forge not handling dispencer behavior.
It needs to be handled by Forge, but i can easily create temp fix for it - since you can write your own dispencer behaviors(as i discovered last time).

Hmm, we can place fluids from our buckets using new behaviors. But you can't change empty bucket's behavior, so it's on Forge side to use FluidContainerRegistry in behaviors.

@anti344 I'm assuming that you're planning to work on this one, right?

Of course. It's one little class + adding it to BlockDispencer.dispenseBehaviorRegistry