I take it its external?
Put some water in the outside box of your overflow. If you have a piece of airline handy shove it in the U tube until it hits the middle of the U. Put the U tube in position and suck the air out of the U tube until there is little or no air. Next, plug in your pump and the last of the air should come out of the tube as water flows through it. If the air doesn't go through, you can just jiggle the U tube until the bubbles become dislodged.
Good luck
Christy
