This is a very curious situation. I have a large water tank on the top of a hill. It supplies a gravity fed water system that works fine. There is an under ground pipe with one end A in the bottom of the tank. The other end E is about 80 feet below A, and has a faucet. None of the pipe is above A. If you open E, then water flows out with good pressure as you expect. If E is open and the tank empties, air gets into the pipe at A, and eventually the water stops coming out of E. All seems normal. Except ... If the tank at A is now filled, no water comes out of the open faucet E. What is the simplest shape of pipe that will cause this? I am sure the answer involves vapor lock and the fact you cannot siphon water over a 32 foot rise, but thats it.