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Take a look at a hydraulic bottle jack: there's a very small pump cylinder at the handle, and a much larger slave cylinder at the ram section. If these two were equal there would be no hydraulic advantage. The greater the difference in size between the two, with the master being smaller than the slave, the greater the hydraulic advantage.

Small bore master, easy pedal effort; large bore master, hard pedal effort. It's backwards from what we want to think! Note that a slave cylinder is usually a shorter ram section, and larger in diameter than master. Master's longer stroke and smaller diameter allows the ease of clutch disengagement while still providing sufficient fluid to do the job.
 

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The only American vehicles with a single reservoir master cylinder will be pre-1967. A Federal Law mandated that all brake masters be dual chamber reservoir in '67 and beyond.

The main difference between a clutch master and a brake master, when they're similar bore diameter, is bore depth. A brake master usually has a deeper bore so it moves a larger volume of fluid. You can still get '60's Dodge truck single clutch master cylinders, or brake masters. The hot ticket is to get the right one then have it lined with brass or stainless steel as a lifetime master cylinder. Stock up on a few rebuild kits and never have to worry about it again! I can provide a link for a man that relines masters with stainless over here.
 
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