Water level has to be (1 1/2 the pipe diameter) higher than normal water level to run a full pipe of water with a conventional pond pipe that includes a canopy inlet.
This is optimum conditions to start suction and a pipe has to be submerged 1 and 1/2 times its dia. to start sucking and run a full pipe.
I wouldn't expect the siphon system to start running a full pipe as quickly, because of the incoming water having to pulled through piping and being less available.
I don't think air will be pushed anywhere out of the system. The pressure is on the pond side trying to push into both the vent and system. If the air can't go up it is trapped. The pressure on the pond side is not great enough to force a full pipe of water and purge the system on larger pipes.
