This vacuum pump is a piston pump. With each cycle it removes a smaller number of air molecules until it just keeps up with what is leaking in past the joints and valve seats.

The amount of vacuum at that point depends on the quality of the components -- how fast the valves close, how tight the seals are, etc.

(This is a still representation and possibly also a low-resolution image of a pump which is fully animated in the CD-ROM version of this glossary.)