I see that 6cylindertexas beat me to a link to MPBrakes so I'll add this as seconding his opinion.
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http://www.mpbrakes.com/mc1004.jpg[/img]
Power masters like the one above, usually have very shallow holes in the piston for the rod. As it says, that one is set up for use below the floorboard.
Non power masters must have a smaller piston diameter or you will have to use both feet on the pedal. Non power masters usually have a very deep hole so that the rod does not fall out when you take your foot off the pedal and the spring returns the rod faster than the fluid moves the piston back.
Here is a page from MP with answers to everything you never thought you needed to know.
Master Cylinders
