Brake master cylinder rebuild.
 

Anyone done this before? Any special tricks to it? It seems simple enough I was considering just rebuilding the one I had vs spending the extra for a new one.

its not hard. only thing you typically replace are the cups. You will need a set of snap ring pliers to get the ring out, piston should slide out with the ring gone. pull the cup off the piston, clean it all up, put it back together....if the piston doesnt come out, you might as well think about getting a new part.

Well, if you buy a rebuild kit from mazda does it include a new piston?

usually they dont. Seriously, go get one for 40 bucks from rockauto.com (just checked, if its for the car in your sig, its 40 bucks and has the piston)