Do you mean master cylinder? A larger diameter master cylinder in the same hydraulic system would increase the required pedal effort to stop the car and decrease the braking capabilities. If there is no problem with the master cylinder at all, I don't really see a dire need to replace it.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record