Thermal Capacity

The brake rotors must be capable of absorbing the heat generated by the brakes as they convert the moving car’s kinetic energy into heat. The amount of kinetic energy a trailer has (and, therefore, the amount of heat the rotors must be able to absorb) depends on the weight of the trailer and the square of the speed of the trailer. The rotor’s ability to absorb this heat depends on its mass (weight), and on how well it cools. Exposed as they are to cooling airflow, this is one area where discs are superior to drums.
Recall that brakes need to be able to do 3 things:
  1. Develop enough clamping force to create enough friction between pad and rotor to convert trailers kinetic energy to heat.
  2. Develop enough brake torque to reach the limit of traction (lockup the tires) in all conditions, and
  3. Have enough mass to absorb the conversion of the trailers kinetic energy to heat without boiling the fluid, warping the rotors, cooking seals, etc. And they must do both with good and consistent feel! 

In order to apply the brakes the driver must be able to both move and pressurize the hydraulic fluid. The master cylinder piston has the job of moving the brake fluid through the lines in order to bring the brake pads into contact with the rotor, and of pressurizing that fluid creates clamping force.