my cars brakes don't work so well you can put the peddle to the floor.. mechanic told me it was a master cylinder, has anyone ever changed one before, how hard is it

 

Its easy, take off the two lines, (buy a set of flare tubing wrenches, it really sucks rounding off the fittings), pull two nuts off that hold it to the booster, remove the bracket that holds the Proportioning valve in place and take it off. Bench bleed the new one and install, fairly easy...