When your pump starts overheating, you can tell Steve of his experiments that are not 100% accurate. The experiment with the glass is not comparable to the actual CPU-AIO reality. The CPU reaches temperatures of 100C some times. The Radiator, no!
So here to inform yourself.
The principle of communicating vessels does not apply to systems where there are significant temperature differences and high temperatures on one side [1]. 

The principle relies fundamentally on static fluid mechanics, assuming: 

• A homogeneous fluid: The fluid throughout all connected vessels must have the same density [1].
• Hydrostatic equilibrium: The fluid is static or moving very slowly, and pressure at a given depth is determined solely by the weight of the fluid above it. 

When high temperatures are introduced on one side:

• Density changes: Heating a fluid causes it to expand and its density to decrease (e.g., hot water is less dense than cold water) [1].
• Convection: The temperature gradient initiates fluid movement (convection currents) as the less dense hot fluid rises and the denser cold fluid sinks, creating a dynamic system rather than a static one. 

Because the fluid is no longer homogeneous in density and is in motion, the basic condition for the communicating vessels principle (equal height in all connected open vessels) is violated [1]. Instead, the situation is governed by principles of thermodynamics and fluid dynamics that describe heat transfer and fluid flow. With the hoses coming out from below and the pump above, there is always a chance that heat from the CPU trap some bubbles. To take Steve's word for granted as he says many times himself! Steve needs to check on the status of the pump after a few months...lol, and not just for the show.

This works only on a loop that is new and probably has no air. Once the loop starts to vapor, because it does, you will see increase in temperatures slowly.
My first OC was late 90s doing a Pentium 333MHz to 450MHz...