Do siphons really require air pressure to function? Can the siphon mechanism work in vacuums?
There seems to have been experiments where exotic liquids (low vapor pressure) were used in siphons mounted in vacuum chambers. The idea is that the falling section of the liquid literally pulls the rest of the fluid channel using its weight via intermolecular forces--equivalent to the fluid having a tensile strength.
Whereas when we observe normal siphons we are quick to attribute the continuous flow to air pressure which pushses up the rising section.
 A: There is a video on YouTube of such an experiment. The corresponding paper is as follows:
Can a Siphon Work In Vacuo? Adrian L. Boatwright, Simon Puttick and Peter Licence. J. Chem. Educ., 2011, 88 (11), pp 1547–1550. DOI: 10.1021/ed2001818
After watching the video (but not reading the paper), my first thought was that the liquid in question has unusually strong intermolecular forces. If it didn't then it wouldn't be able to remain a liquid in such a strong vacuum. This must mean that this liquid can be held under a much higher degree of tension than something like water. 
This means that this experiment can't rule out the possibility that air pressure is important for the operation of a siphon when water is used. And in fact, the air pressure does play an important role in a normal water-based siphon: it prevents the water from immediately boiling away. At zero pressure water turns into a gas and has no tensile strength at all, so we can say that the surrounding air pressure is necessary to hold up the water inside a normal everyday siphon.
A: Yes,
they can. But for a short time only, if the forces 
applied are stronger than gravitational force. 
The cohesive forces of liquids are quite impressive, 
but following a slight "cut" from the side or inside 
the column, it will disrupt. 
Of course You have to free the liquid of dissolved gasses 
first, and the tube must not have sharp edges or "dents".
In vacuum practice You have siphons within mercury or oil 
diffusen pumps for example. Any U-Tube manometer is such 
a siphon. 
Georg
