http://www.boatus.com/boattech/casey/13.htm
The thing I was a bit confused on was where the vented loops had to go... and I see by the article, that they belong after the pump. So obviously you cannot "draw" on a vented loop, but you can "push" through it.
Casey also suggests that you may not need one on the input side if you have a valve... I have a feeling that is a very touchy "may" situation.