Block enity slot automation and hoppers

I am trying to experiment with a block to store liquids. Transfer to/from buckets. 2 slots, one for deposit and one for withdraws. However, when I place the item into either slot (with a hopper under the block) it takes them out before the logic has a chance to trigger! I am lost on how to start with the procedure for telling the hopper to not take an empty bucket only from the one and a full buck from the other. Can anyone point me in the right direction? TIA!!!