Definition of a Function. A function is a relation for which each value from the set the first components of the ordered pairs is associated with exactly one value from the set of second components of the ordered pair. Okay, that is a mouth full.
Master cylinder without booster. Single bore master cylinders: These unboosted hydraulic Master Cylinders are actuated by foot effort, via the brake pedal. The brake pressure is related to actuation force and depends on the piston bore size: the smaller the bore, the higher the pressure.
A power booster uses atmospheric pressure to help assist the pushing of the master cylinder piston. Basically, a power booster is divided into two chambers internally with a rubber diaphragm seperating each. With no force applied to the booster there will be vacuum on both sides of the diaphragm.