i fill both sides with water and get a piece of flexible air line tubing stick it in one end to the top of the U and start sucking...make sure the return pump is on. pinch the air line and pull it out.
oh mine has a seperate chamber for in one side for a constatne water level in the back. you might have to close up the outlet fill the drain tube and then a little of the over flow box
You should also make sure that if you do lose siphon that your display tank will not overflow and inversely that if your return pump goes out your sump wont overflow. Just set your water levels accordingly in your sump and DT to avoid any floods