If a master cylinder with a bore diameter of 2.5 cm is replaced by one with a diameter of 5.0 cm, what is the expected effect on system hydraulic pressure for a given pedal force?

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In a hydraulic brake system, the pressure produced in the fluid for a given pedal push depends on the force applied divided by the piston area of the master cylinder (P = F/A). Increasing the bore diameter from 2.5 cm to 5.0 cm massively increases the piston area (area grows with the square of the diameter). With the same pedal force, this larger area yields a much smaller pressure—specifically, the area becomes four times larger, so the pressure drops to about one quarter. The larger bore also moves more fluid per stroke, so you’d feel more pedal travel for the same braking effect. The type of brake fluid doesn’t change this pressure-forces relationship.