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Well if a molecule getting trapped behind a pump is random, How likely is a molecule to randomly fly into a pump? That depends on how big the tube is leading to the pump.
Tube Size and Conductance
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The trouble for lowering pressure comes when a given type molecule jumps around enough to raise the pressure, but not enough to easily find it's way into the pump. Check out the Application of Vacuum Theory page to see how we deal with troublesome molecules.
Here is a presentation created by Dan Peterswright that introduces some vacuum concepts and walks through Gas Laws. It also offers a few examples that illustrate the effects of pressure differentials created by establishing vacuum in flexible couplings and brief examples that illustrate the related gas laws.
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