I have two 12" siphons with 4" vents. I have seen the vents a good 6" below the surface with a lot of water coming out the 12" pipes, but have never seen it actually siphon. I took to cap off of the tee in the vent so I could see straight down to the 12" pipe, and it had about 6-8" of water flowing over the bend. I am not sure why it didn't siphon.

Hey Moe, I'm trying to think but nuthin's happening!