Pond Boss Forums POND CONSTRUCTION Building a dam Siphon Issues

Hey Guys,
I am new to the sight and am looking for some insight into a problem that we are having with our lake siphon drain. We need to lower the height of the lake water level so that we can make some concrete repairs. We have been drawing it down 12" at a time (week), which is about 3 days of running with full flow on a 6" PVC pipe. This has been okay up until now but we are now about 36" lower than normal level and I can't get it to go. Normal water level is at 1000' above sea level. The inlet pipe is at 989' with an open end. The outlet elevation is at 975' with a gate valve near the end. Vent pipe is near the peak on the drain side pipe at 1000' elevation. Drain side pipe is about 300' long and the inlet pipe length is about 80'. Normal procedure is to remove the cap from the vent pipe, pump water into the drain side pipe all the way up, put vent cap back on, and open the drain valve wide open. You usually see the water drain for a period of minutes and then some air come out, which was present between the vent pipe and the lake, and then water from the lake coming behind it. Now we see a similar thing except the water just seems to die off after the slug of air comes through. I can't detect any air leaks but they could be present somewhere. I did see that some folks have a turned up 90 degree elbow at the end of the discharge pipe. Could this be the issue? Would air be getting sucked up from the outlet end and breaking the suction? The amount of air between the vent pipe and the lake increases every time we do this. Is this creating a problem? Thanks

Do you have a shut off valve on your vent pipe? If not the siphon will not start or stop.
Heppy