Clearance Calibration
When the hydraulic pressure is applied to the piston (3), the piston moves leftward. The piston seal (2) exerts pressure against the piston and moves with the cylinder.
However, a part of the piston seal is fixed into a groove within the cylinder. The shape of the seal is distorted toward the direction the piston is moving, as shown in the figure.
When the pressure is taken off the brake pedal and the hydraulic pressure is released from the piston, a restoring elastic force is generated at the seal. This force pushes the piston rightward and back to the original position.
As the pads (1) wear away and the clearance between the rotor and the pads becomes larger, the piston moves farther.
The shape of the seal could then become more distorted. The distortion is limited to the same amount as previously described, because the end of the seal is fixed into the groove in the cylinder. The piston moves farther to cover the distance of clearance. As the piston returns by the same distance, the rubber seal recovers its original shape, and the clearance between the rotor and pads is maintained in original condition.