Ok, Rainman, it looks like this is more complicated than I thought. I have had a couple restarts since my last post. Even with the elbow at the bottom it still won't seem to flow very long. Maybe it's because of atmospheric pressure changes...?
On my first attempt after adding the elbow, it flowed for over 48 hours and looked like it was going to draw all the way down to the inlet. Then overnight it stopped when presumably the pond was almost done draining, judging by how it looked in the evening. But naturally, after it stopped siphoning that night we got torrential rains and the pond was almost full again by morning.
I restarted it and it flowed for almost 24 hours, dropping the pond level about 6 inches. Restarted again yesterday, and as of this morning dropped maybe another 6 inches. Could my fits and starts be the result of atmospheric pressure changes? My outlet is definitely a few feet below the inlet level, but not much more than a few feet.
Thank you all for the great help and info!
The atmospheric stuff was just for the techie's needs.
How far below the water surface is the inlet? Is it creating a whirlpool in the pond that could allow air to be pulled into the siphon tube? Only 3 things can stop a siphon flow once started...Air entering the tube, a blockage, or water levels equalizing.
Is it possible your pond bottom is deeper than the height of your outlet? I would also suggest using electrical tape to help seal every joint.
The high point of your outlet needs to be about 4 feet lower than the level of your pond water and your inlet must always be submerged in water. If a whirlpool is forming over your inlet, air can be drawn down the center of the whirl over a couple feet deep.
Did you put a screen of some type on the inlet? If the inlet is getting plugged by a fish or other debris, tiny air leaks could be drawn in an breaking the siphon also. I use a capped length of pipe drilled with hundreds of smaller holes (1/2" holes in 2" pipe and 3/4" holes in 4" or larger) to reduce inlet velocities, preventing whirlpooling and preventing clogs.
