Brake Master Cylinder Help

My understanding is as follows;

Using the example of 19 x 20 master cylinder, the 19 is the bore of the cylinder in mm and the 20 is the length of the 'lever' in mm i.e. the length from the pivot point of the lever to the point that pushes the piston into the master cylinder.

An RCS master has a switchable ratio, in the example of a 19RCS it is 19mm bore but with two switchable pivot lengths of either 18mm or 20mm.

Using the RCS on the 18mm gives more travel of lever vs piston whereas the 20mm gives less travel but with less effort required. So on the 18mm setting you need more lever travel and more effort but this gives more feel vs. the 20mm which requires less travel, less effort feel but ultimately less feel to achieve the same braking power.

 

This may help but it says what Dukedesmo said already DUCATI | 999 | 999 | 2003 | Master cylinders | Motorcycle Brakes Configurator from the rcs link, it says the following for a 19RCSLL
Radial master cylinder designed for street bikes that allows maximum readiness and perfectly modulable braking in any conditions of use with excellent linearity between the force applied to the lever and the braking response.
This brake master cylinder has the RCS system, patented by Brembo, for adjustment of the lever centre distance which allows you to obtain the ideal feeling between rider and bike. In just a few seconds using a simple screwdriver you can make the braking system more aggressive and reactive (20 mm) or softer and modulable (18 mm).
This model uses a 19 mm diameter piston and an folding long lever (LL), a detail which is particularly popular with Racing riders and those who love to ride on the track.
Brembo uses only cutting-edge procedures in order to guarantee excellent operation and performance. In fact, the aluminium alloy brake master cylinder main body is obtained through forging and then machined with numerically controlled equipment.
For the surface finishing oxidation was chosen which is normally used in MotoGP and Superbike.

 

By the way the RCS range has a 2mm variation between the 2 ratios; 19RCS is 19 x 18/20, 15RCS is 15 x 18/20 and 16RCS (clutch slave is) 16 x 16/18.

Presumably because the eccentric for the 'plunger' has a 1mm offset so that rotating it 180 degrees gives a 2mm shift.