A liquid UNLOADER should unload carts I think? If it has a black square on top it is an unloader and needs to be placed under the track to unload the cart. A liquid loader must be placed above the cart to load it and does not have a black square on top.
EVE Online characters:
Daedalus II ex-CEO TOGOA, Daedalus III slave of Daedalus II
Little Molok Asura Engineer
"Trying is the first step towards failiure" - Homer Simpson