I have a 4x12 GB with a 2" standpipe, 4" bell. Siphon is at one end and inflow is at the other. Siphon starts but stops before all the water at the other end of the GB has a chance to drain out. I do have a breather tube and it just gurgles while the water at the opposite end is trying to drain. Any ideas on how to get the entire bed to drain? Should the siphon be in the middle of the bed?