Affter a little testing with sorting buckets, i found dispencers remove the lava from a bucket and don't place a scouce block when facing a block that could take a water souce block leaving behind an empty bucket. In all other cases the lava bucket is dispenced as intended. Only seems to affect lava and not anything else in a bucket. I assune this isn't as intended as using the lava bucket on a water logable block just places the source block on the side closest to the player.

Set up is simple:

1.Place dispencer.

2.Place iron bars infront of dispencer.

3.Place lava bucket in dispencer.

4.Attach lever to any side of dispencer. 

5.Flip lever.