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.
The brake booster usually uses vacuum from the engine intake to boost the force applied by the pedal on to the master cylinder, or may employ an extra vacuum pump to enable it. Without the engine running the brake pedal feels very hard and ineffective on the braking capability.