After re-reading your post, I see you want to totally drain the pond. You could still use the method I just said, but use your rubber collar adaptor and slide the tube going into the overflow through it so it will be below your water level. Cap the outlet of your overflow and use the U-Tube to fill your overflow. Once it is full, you can add length to your pipe on the pond side to get it on the bottom. As long as the pipes are all sealed, even a moderate amount of air being in the pipes at the start won't effect the siphon once it starts.

How deep is the pond? A siphon can only work at a max of twenty feet. Also put some sort of support under the short piece of PVC where it goes over your stand pipe, and/or use schedule 40, because once the level drops considerably, that pipe is going to get damned heavy and may even snap S & D pipe.