What is a disc brake
A disc brake has a disc that turns with the wheel. The disc is straddled by a caliper , in which there are small hydraulic pistons worked by pressure from the master cylinder.
The pistons press on friction pads that clamp against the disc from each side to slow or stop it. The pads are shaped to cover a broad sector of the disc.
There may be more than a single pair of pistons, especially in dual-circuit brakes.
The pistons move only a tiny distance to apply the brakes, and the pads barely clear the disc when the brakes are released. They have no return springs .
Rubber sealing rings round the pistons are designed to let the pistons slip forward gradually as the pads wear down, so that the tiny gap remains constant and the brakes do not need adjustment.
Many later cars have wear sensors leads embedded in the pads. When the pads are nearly worn out, the leads are exposed and short-circuited by the metal disc, illuminating a warning light on the instrument panel.
