Say you have a master piston size of say (because I dont know) 15mm diameter. This pushes ONE piston of diameter 15mm similar to how a BB7 works. This gives a master/slave ratio of 1:1, obviously. Now if we say the 15mm master pushes TWO 15mm diameter pistons, this can be equivalent to a single piston of a larger size (NOT 30mm diameter btw) and thus gives a greater force applied to the pads/rotor 1.76714 * 10-4 : 1.76714 * 10-4 Is one master to one slave 1.76714 * 10-4 : 3.53428 * 10-4 is one master to two slaves. This is equivalent to a 15mm diameter master to ONE 21.21mm slave Now I think you've grasped this bit, its for those that hadn't. What you're saying is if you had one slave or two slaves, if the pads were the same size, why is there a difference in friction/force? Well first of all, don't confuse force with pressure: Pressure = Force / Area Force = Pressure * Area Hydraulic fluid exerts pressure onto a piston surface area, which creates a force. So the piston pressure is the SAME in the same sized 4 pot/6 pot brakes, but the fact that there are more pistons, this INCREASES the area and as you can see from the formula, INCREASES the force. So, although you're pulling on the lever with the same force, you creating the same pressure in the fluid, which creates the same pressure on the pistons, BUT because there are more pistons/larger piston area, you're creating a larger force! Hope that helps