How much rise vs fall do you have?

Also I would check all the joints for leaks, I had a femco that wasnt tight enough on my siphon stop it one time.

Here is a thread on my pond project where you can see my siphon last summer when I drained it.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=30089&Number=390173#Post390173

My pond renovation thread here