Dave, squeeky is correct, but maybe if you keep in mind the pipe going down the backside of the pond will always be longer (to give it the 4 ft lower needed to work correctly) than the pipe going down the front side it may help. The water filling the pipe going down the backside of the dam will always have enough weight to help in the lift of the water in the front side of the pond.
If the pipes were equal in length they wouldn't produce a siphon effect. hope this helps.
