OK, I'll do my best. Siphons work on differential pressure where that differential pressure was once sufficient to overcome gravity and "start" the siphon. Somehow you had to get fluid to the top of the 1/U and get the pressures balanced. You had to fight gravity by "sucking on the end" and getting a mouthfull of gasoline, or you had to put a submersible pump in the tank and pump it up to the lip, over the top and down the downcomer on the outside. Once all pressures are exactly balanced, only a slight perturbation will allow them to continue. That perturbation is momentum, and some surface tension or capilary action if small diameter piping and maybe some kind of unknown (to me) "stickyness" that fluid seems to have, that pulls a little bit of the trailing fluid along with the moving fluid. Not my description, that came from the Physics guys. When you look at it, you see that the fluid, once started flowing, continues to flow, seemingly without a running power source, and its a mystery, but obviously there is still something powering them.
To start a siphon, you must have a differential pressure to raise the fluid to the top of the 1/U. To do that, you must fight gravity for the run up the riser. Once you get to the top and start to go over, gravity helps on that bit as it starts to move down in the downcomer, but there's still a lot of fluid in the riser, where gravity opposes the flow direction. "If you stop sucking, the gas goes back into the tank". At that point you've only cancelled out a little bit of gravity. As the fluid on the downcomer side gets lower, gravity difference between the riser and the downcomer is more balanced and when the fluid in the downcomer finally gets all the way down to the level of the surface inside the tank (ambient pressure on both liquid surfaces (inside the tank and the liquid surface at the interface inside the pipe, being equal), gravity and pressure are completely balanced. "stop sucking now (almost)." If the pipe ended at that point and was completely open to gravity, the fluid in the downcomer would simply tend to fall out. As it fell out, it would evacuate the downcomer which would tend to reduce the pressure at the top of the "1/U", that was just before completely balanced. That slight pressure difference across the top of the U would allow a little bit of fluid in the riser to flow over the top into the downcomer. The faster the liquid fell out of the downcomer, the faster the fluid would also move over the top. The fluid in the riser is no longer balanced because its lost some of the weight of the fluid at its top, so the pressure in the tank is again sufficient to push a little more fluid into the bottom of the riser and force the remaining fluid in the riser to move slightly higher, once again balancing pressure at the top of the "1/U".
Take the example you mention for the evacuated tank. Think in terms of absolute pressure. If internal pressure is 5 psia, and the siphon is open to atmosphere somewhere, you've got 15 psi pushing in and only 5 pushing out. Let's say we ditch the 1/U for a, /\/\ where the left "/\" is full of diesel and the right "/\" is full of air, and we let the fluid oscillate back and forth for a minute or two and see what happens. Eventually, the "\/" in the middle will reach a point where it has apx 21 more feet of diesel in its left leg than it has in the right leg, oscillations will stop and all flows will halt, because the 10 psi differential pressure would be balanced by the 21 feet of differential head of the diesel in the left leg over that of the right leg. You'd have to connect a "vacuum pump" to the right leg's outlet to get any more diesel out of that tank.
Take a look at Bernoulli's equation and see if it makes sense now. Flow Energy at any point = elevation + Pressure head + velocity^2/2g - Head loss due to flow. If not, see if you can find out how a mercury barometer works. A water barometer is the same except instead of 760 mm, its 33 ft, and a diesel barometer needs 40 ft.
And one more thing... remember that in a closed tank, with just one drop of fluid remaining inside, the absolute pressure in that tank is equal to the vapor pressure of that fluid and that's as much vacuum as you can ever get in that tank at that temp.
Don't feel you have to make excuses. I can't understand how to wire a 2-switch-at-each-door-going-to-the-one-light-in-the-middle-of-the-hallway to save my life.
