I'm testing my first system with a grow bed about 20 inches deep, a 1/2 inch stand pipe 18 inches tall. The bell siphon starts OK, but shuts off with about 6 inches left in the grow bed....meaning, the siphon is not lifting the water more than 12 inches above the water level in the bed. Does anyone have a siphon that is taller than 12 inches?

From what I read, the problem may be with the exit piping. I only have about 3 inches below the grow bed, then an elbow, then another 3 inch horizontal pipe ...all of these are 1/2 inch pvc. The horizontal pipe ends inside a 3 inch master drain pipe.

I'm looking for any suggestions to get the grow bed to drain fully.
Thanks!