does anyone have some places where i can get a thumb brake? Thanks
Yes, a larger master moves more fluid... But a smaller master attains much higher peak pressures because you are applying the same hand pressure onto a much smaller area. That means More Power, but (obviously) to move the same amount of fluid you need a longer stroke. That is why M/C brake pads/pistons are designed to use as little travel as possible.
Now, the cool thing about hydraulics is that ALL PRESSURES APPLY EQUALLY IN ALL DIRECTIONS, ALWAYS. The system is either pressurized, or it is not.
So how it all works is that the force applied by your hands gets multiplied, first by simple leverage of the lever on the master, but then hugely multiplied by the relative surface areas (master:calipers). For example, if the diameter of the slaves is twice that of the master, then that multiplication factor is roughly 4:1. Oh Yeah, I am greatly simplifying the math, so you engineers can please not bother to correct me....
Now, our brakes would have that 14mm piston pushing something like four 40mm pistons on the caliper end. An area of (about) 44mm pushing on (about 125, x 4) 500 sq mm. 11 times the surface area...
So, if you applied 10 lbs of pressure, 5 inches out on the lever (50 in/lbs), through the above setup, the pressure at the caliper's pistons would be (50 x 11) 550 lbs, per square in. Which in turn is multiplied by the surface area of the pads. If we call the pad's surface area 1.5" x 3", that's 4.5 square inches, or 2,475 pounds worth of pressure on the brake rotor.
This, of course assumes no flex, no expansion in the brake lines, and no friction in the system and perfect contact between all working surfaces.
Take a 16mm master cylinder (50 sq mm) through the same process, and you get (50 x 10 x 4.5) 2,250 lbs sq/in. A decrease of 225 lbs worth of pressure for a 2mm increase in master cylinder size.
The issue with a smaller master, is that at some point the lever travel necessary to build pressure becomes excessive. That defines the practical lower limit. Conversely, a larger master feels more immediate, since it moves more fluid and builds to it's (lower) peak pressure sooner. But losses at the master also get multiplied down to the rotors....
That was why there was the push for 6 piston brake calipers for a while. It was an easy way to get a lot of force multiplication. The problem with that was that a lot of the force was lost through flex. So we went back to beefier and less flex prone 4 piston calipers, pushed by smaller master cylinders.
The other things we learn is that a couple mm worth of difference at the master means a BIG difference at the rotor/wheel. So we should take it easy when we start d*cking around with modifications. And if you were really paying attention, you would have figured out that it's a good idea to move your brake lever as far inboard as you can, so you'll grasp it further out and get a little 'free' power from your brakes.
After all, if you grabbed the first example's lever an inch further out (60 in/lbs applied) then your brake rotor gets 2,970 pounds of pressure....
Now that I've bastardized it all to hell and back, if anyone wants to really know the technical detailia behind all this stuff and don't mind Math for Engineers, then pick up a copy of "Motorcycle Tuning - Chassis" by a gentleman named John Robinson.