brake booster...what is it?

[uiuc240]

OK, so the brake booster. The large, round, black thing that your master cylinder hangs from. What is inside that?

I know that it takes vacuum from the motor and converts it into a useful boost in braking power...but how? What's in there? Are there any moving parts? Is there anything that can actually wear out? Is there anything rebuildable?

I'm asking because my pedal feel has slowly been getting "crunchier" ... if you can imagine that. I don't know if it's something caliper-related...but I'm trying to rule out other things first. The master cylinder is a reman, but only aobut 3 years old. The Z brakes are old (1990), but they were rebuilt when I put them on the car 3 years ago.

I know I need to pads and rotors, but I think the "crunchy" feel I'm getting is something else.

Any ideas? (Calling all brake gurus!)

Eric