You guys helped me realize something. The Python vacuum pump works because of Bernoulli's Principle. This principle (much simplified) states that: When the speed (or kinetic energy) of a fluid INCREASES the pressure DECREASES. This is the reason for the narrow spout inside the python pump that looks like this:
Water being forced through this spout causes an increase in the speed of the flow, therefore causing a low pressure zone inside the pump. Since fluids (such as air and water) naturally flow from High to Low pressure the water flows from the tank into the pump.
All of this was included in my home-made design using a modified water balloon filler spout that looks like this:
Yet my design was a complete and total failure. Something else is required to make the python work. This:
This is the drain valve. It also restricts the flow, sealing the bottom of the pump when water is run through it, which prevents high pressure air from coming back in the bottom of the pump. Naturally, air is easier to move than water, so if this isn't present to prevent air from coming in the bottom, the pump won't siphon water. If you take this valve cap off, the python pump WILL NOT WORK. Just like my homemade one did not. I could actually make my homemade pump siphon water if I wrapped my hand around the bottom a certain way, it just wasn't reliable enough to be useful.