| Let me answer the part about braking power first. Changing your stock master cylinder to an aftermarket radial design will NOT give you more braking power to help you stop sooner.
A radial master cylinder with a different piston diameter and/or lever fulcrum-to-piston distance will only change the feel of the brakes at the lever. So keep in mind that Ducati chose a master cylinder size to give you the best modulation characteristics (feel, sensitivity and control) for your bike.
Good modulation means good feedback to the rider during a stop. A good braking system needs to establish the closest linear relationship possible between the force applied to the brake lever and the actual deceleration of the bike. Stopping power is technically easy to achieve, but achieving it along with good proportional braking response is more difficult. This, I feel, is the major factor influencing braking quality.
That’s not to say that the Ducati engineering department’s choice is best for all riders or riding conditions. The best choice for the track isn’t the best choice for the street.
Different riders have different preferences and we’re all adaptable. A rider is able to compensate for one performance drawback to gain an advantage with another. But again, it's situational dependent; a braking system that gives repeated stops from 150mph with the force application of one finger is not necessarily optimum for a 40mph panic stop in traffic. Even though a rider is adaptive to a braking system's general behavior doesn't mean that in an emergency that he'll use a light one-finger pull to stop.
So let’s move on to your choices.
First, there’s a different master cylinder size requirement for single rotor systems than for dual rotor systems. A dual rotor set-up has a lot more caliper pistons to move so a larger volume of hydraulic fluid has to be moved by the master cylinder piston.
Also, since different Ducati models have different size calipers and rotors you can’t always translate a recommended master cylinder size to another setup.
A master cylinder size designation is written AAxBB where AA is the diameter of the piston in mm, BB is the fulcrum-to-piston distance in mm.
The master cylinder piston diameter is chosen based on the number and size of the caliper pistons.
A fulcrum-to-piston distance affects two things: the amount of force needed at the lever, and the distance that the lever needs to be pulled through (to displace and compress the same amount of hydraulic fluid which in turns forces the caliper pistons against the rotor discs) to yield the SAME stopping power.
Single Disc
The Brembo aftermarket radial 16mm diameter master cylinders offer you the choice of a brake lever fulcrum-to-piston distance of either 16mm or 18mm. The stock Brembo lever has a 16mm dimension.
So your choice is either 16x16 or 16x18.
From the geometry, a 18mm lever will need to be pulled a 11% shorter distance than a 16mm lever but will at the same time require more lever force than a 16mm to stop the same distance.
Dual Disc
The Brembo aftermarket radial 19mm diameter master cylinders offer you the choice of a brake lever fulcrum-to-piston distance of either 16mm, 18mm or 20mm. The stock lever is 16mm.
So your choice is either 19x16, 19x18 or 19x20.
19x16 (stock) requires the least less lever effort but the longest pull distance.
19x18 requires 11% lighter pull and 11% longer pull distance than the 19x20 MC. More feel (better modulation characteristics) than the 19x20
19x20 requires the most lever force but the shortest pull. More like a trigger action.
Some riders think that this short-pull trigger action means that they have "better" brakes, but they don't - at least not for all riding conditions. What they do get is the same braking power with poorer modulation (feel) characteristics. Good for the track perhaps, but often dangerous on the street, especially in the wet. In an emergency, most of us have the instinct to grab a brake hard. So if you value a better feel, when choosing between the 19x18 and the 19x20 for the street, go for the 19X18. Better still stay with Ducati’s choice, 19x16.
On the other hand, some prefer their lever hard.
So again, I'm not suggesting that every rider will have the same preference in a braking system's modulation characteristics. Depending on your preference (or need) you can have brakes with an initial vague feeling, a strong initial bite, or something in between. You can select pads that have better high temperature behavior. On a race bike you can select brake pad and rotor material that will survive a race without needing replacement, but on the street, materials need to be more durable and function under less severe braking conditions and more varied weather conditions.
Master Cylinder Piston Diameter Selection
Brembo makes 16mm diameter piston master cylinders for use with single caliper brakes and 19mm diameter for use with dual calipers. Why? To give the proper modulation characteristics at the lever. The different diameter MC pistons change the hydraulic relationship between the master cylinder and the caliper pistons.
The hydraulic advantage is the total area of the caliper pistons divided by the area of the master cylinder.
For example:
The Brembo Goldline front caliper has four pistons. Two are 30mm diameter, two are 34mm.
The Brembo Monoblock front caliper has four pistons. Two are 32mm diameter, two are 36mm.
Case 1: A single Goldline actuated by a 16mm MC.
2(30 x 30 + 36 x 36) / (16 x 16) = 2(900 + 1296)/256 = 17 >13
Case 2: Dual Goldlines actuated by a 19mm MC.
4(30 x 30 + 36 x 36) / (19 x 19) = 4(2196)/361 = 27 < 24 > 23 (firm)
Case 3: Dual Monoblocks actuated by a 19mm MC.
4(32 x 32 + 36 x 36) / (19 x 19) = 4(1024 + 1296)/361 = 27 < 26 > 23
It has been my experience that there is a "sweet spot" in the range. I like ratios in the 27:1 range-2 finger power brakes, feeling some line and/or caliper flex.
23:1 is at the other end of the spectrum-firm.
Case 4: Dual Goldlines actuated by a 16mm MC.
4(30 x 30 + 36 x 36) / (16 x 16) = 4(2196)/256 = 34 > 27 (loose)
Case 5: A single Goldline actuated by a 19mm MC.
2(30 x 30 + 36 x 36) / (19 x 19) = 2(900 + 1296)/361 = 12 < 23 NOT GOOD
Ratios lower than 23:1 produce a lever feel so "wooden" as to have little, if any feel.  |