Brake Pads, Disc and Piston

Brake pads should be replaced every now and then, the minimum your pad can be is 1mm, so have to keep a look out for that. You don't want it to completely ware off and damage your brake disc.

BRAKE DISC
The brake disc is the disc component of a disc brake against which the brake pads are applied. The weight and power of the vehicle determines the need for ventilated discs, The "ventilated" disc design helps to disperse the generated heat for faster cooling. Discs are usually damaged in one of four ways, scarring, cracking, warping or excessive rusting.

RUN OUT and THICKNESS OF DISC
The Run out is measured using a DTI gauge, once its installed right, spin the disc and measure the maximum and minimum run out, if its out of the manufactures range then it can be machined.

Thickness of disc can be measured by using a micro meter, you need to measure the inner thickness and the outer thickness, and if the difference is too much then it can be grinded down. 

SLIDES and THE PISTON

The movement of the pistons is controlled by a hydraulic system. When hydraulic pressure is applied by pressing the brake pedal, the piston is pushed forward to press the inner pad against the rotor while the housing is pushed in the opposite direction to press the outer pad against the rotor, hence generating a hydraulic clamp around the rotor. For the fixed (non-floating) caliper type of disc brake system, each piston presses the brake pad against its respective side of the brake rubbing surface, as shown in Figure