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Sadly adding a bigger pump to suck faster though the tiny tube only works in viscous flow. In molecular flow, the pump will only ever remove molecules at the rate they naturally fly through the smallest/longest tube in leading to the pump.  The lowest conductance point sets the pace.  =(

Mixing Gasses: Partial pressure

So gasses mix. How does mixing gasses change pressure? The pressures add up. Here's an example:

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In the mixed chamber the oxygen and nitrogen are still contributing the same pressures they had when they were separate. Those contributing pressures are called partial pressures. Oxygen's partial pressure in the mixed chamber is 2 units of pressure.

Stuck on walls: Condensation and Evaporation

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Ok, so you know how I said molecules bounce off walls? Not actually true. I lied to keep it simple, but I think you're ready for the truth: It turns out that any time a gas molecule hits a surface it actually sticks to that surface, just like how your breath fogs the mirror; this called adsorption (not to be confused with absorption).

And also just like fog on a mirror vanishes over time, those stuck molecules will eventually jump back off of the surface (I.e. evaporates, outgasses, desorbs), flying of in a random direction just as fast as it was when it hit the surface.

How long does the molecule stay stuck before desorbing from the surface? Well that depends on the molecule, the temperature, and the what the surface is made of; and it can happen anytime between almost instantly and basically never.

If a type molecule tends to jump off quickly, from a given surface at a given temperature, it will find it's way into a pump pretty quickly, and the pressure will drop. And if a type molecule tends to jump off rarely, from a given surface at a given temperature, the pressure also goes down, it's not even contributing the the pressure, because it's not a gas, it's trapped on the surface!

The trouble for pressure comes when a molecule jumps around enough to raise the pressure, but not enough to easily find it's way into a pump. 

contamination: molecules sticking to walls intermittently

main culprits:

water - from moisture in the air 

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