Pond Boss Forums GENERAL QUESTIONS Help Siphon drain

I am assuming that since the dive team plugged the outlet, and a siphon started after plug was pulled, that the diver did not find any inlet blockage??? That leaves only one other possibility....that air is entering the system. I also assume the "90" on the outlet is pointed upward and that there is a constant pool of water in the bend of the elbow that blocks air from entering the outlet down pipe? If so, I would start electrical taping/regluing every joint first before chasing cracks. The vent pipe is what I would inspect first and most closely as people/critters tend to pull on it. Had one pipe I guessed a deer tried jumping over and hit the vent, cracking the PVC and breaking the siphon.

Place something OTHER THAN YOUR HAND, like a strip of thin cardboard, over the vent tube inlet under the water to see if there is a considerably strong vacuum/suction....if not, there is definitely air entering the pipe. (cereal box cardboard will break down quickly once wet so no extra blockage worries)