Has anyone ever cracked open a PP master? I took apart my stock one and it has two pistons in it , and a little floating divider. Maybe I am over thinking it, but I dont see why they couldnt have two different bore / piston sizes in the same master...TL;DR: Master cylinder piston area to caliper piston area controls pedal effort, height and feel. If we keep all else the same, though, it doesn't influence actual braking capacity or bias.
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Let's look at these two master cylinders, 23.5mm vs 22.0mm:
That's a 14% difference on the master cylinder piston area, which means a 14% difference in braking force for a given pedal pressure. It also means a greater force change with your foot on the pedal to modulate.
- Pedal pressure for a given amount of braking would be 14% higher for the 23.5mm master than the 22mm one.
- Pedal *travel* will be longer with the smaller master. As long as nothing gets into a bad situation geometrically or with respect to travel (i.e. it doesn't bottom out or put the pedal in an over center position), it will not affect performance.
- Larger MC's (with greater effort) generally get better comments concerning modulation
- Larger MC's will have a higher and firmer pedal
- If the master is too large, then the driver might struggle to reach an appropriate peak pressure or may just have to use too much muscle to effectively drive.
- A standard automotive master (as opposed to dual masters on a race car) doesn't change F/R bias.
- The ABS controller regulates proportioning/brake bias.
- The brake booster has a big effect on pedal feel, but it doesn't sound like anyone is changing boosters.
- The staggered piston sizing of the Shelby is to reduce taper wear on the pad. Not having it on the PP brakes is just an insult. Calipers with staggered pistons are no different cost than with uniform piston sizes. It's just an effort to give the Shelby a talking point by tanking the PP.
- Pad compounds can have a much bigger effect on max pedal pressure and modulation than MC sizing.
- A big problem when testing master cylinders is getting an effective bleed. It's damned tough to beat the factory bleed, so don't crack the lines unless you have to. If you changed a master cylinder and found bias or performance differences, then it may very well be a bad bleed as opposed to a problem with the master sizing.
Just a thought as to why they would have a different master, I don't think pedal effort is the only reason..