Your drain pipe should always be big enough to be able to empty the grow bed at least 1 1/2 to 2 times faster than what your input in volume coming in can fill it. Going smaller is going in the wrong direction. In all actuality, you can't make your drain pipe too big because it's all in accordance to how quickly you want to empty the grow bed. Make sure the open end of the "U" siphon is about 1/2 inch from the bottom and absolutely level and parallel to the bottom of the grow bed. In the case of a rounded bottom grow bed, you will have to adjust accordingly because that's what caused the suction to break. I hope this helps. Sit down, kick back and pop a cool one for me.