The disc brake caliper assembly is bolted to the vehicle axle housing or suspension.
There are 2 main types:
Fixed calipers can have 2, 3, or 4 pistons. 2-piston calipers have one piston on each side of the disc. Each piston has its own disc pad.
When the brakes are applied, hydraulic pressure forces both pistons inwards, causing the pads to come in contact with the rotating disc.
The sliding or floating caliper has 2 pads but only 1 piston. The caliper is mounted on pins or bushes that let it move from side to side.
When the brakes are applied, hydraulic pressure forces the piston inwards. This pushes the pad against the disc. The caliper is free to move on slides, so there is a clamping effect between the inner and outer pads. Equal force is then applied to both pads which clamp against the disc.
In disc brake calipers, the piston moves against a stationary square section sealing ring.
When the brakes are applied, the piston slightly deforms the seal.
When the brakes are released, the seal returns to its original shape. The action of this sealing ring retracts the piston to provide a small running clearance between the disc and pads. It also makes the brake self-adjusting.