Released · small clearance at the rotor

Start at the lever and master cylinder

One connected system · the floating controls operate all three views

Bicycle brake lever and master cylinder cutaway A pivoting lever drives a pushrod and master piston. The primary seal passes the compensation port before the pressure chamber drives fluid towards the caliper. A return spring brings the piston back when the lever is released. The reservoir and diaphragm accommodate fluid volume changes at rest. This is a simplified open-reservoir design; real layouts vary.

At rest

Reservoir connected

The primary seal rests behind the compensation port. Fluid can move between the reservoir and the pressure circuit to accommodate temperature and pad wear.

Reservoir connection open

The master-cylinder seal builds pressure. The caliper seals below provide the small piston rollback. They do different jobs.

Typical open-reservoir principle. One compensation passage is shown; refill passages, linkages and seal profiles vary. The diaphragm is a flexible barrier above the fluid, not a pump.

Inside a 4-piston bicycle brake

Two pistons per pad · cutaway through both piston pairs

Four-piston bicycle disc brake caliper Four pistons are arranged in two opposed pairs. Two pistons push each long brake pad towards the rotor. Fluid channels supply the same pressure to all four piston chambers. One annular seal surrounds each piston; a section cuts each ring at the top and bottom. The highlighted seal A is shown in the enlarged view.
Brake fluid Piston seal Relaxed outline

Inside the piston seal

A · Enlarged view of the chamfered groove

Seal flexing into a chamfered groove relief The seal sits in a groove machined into the caliper bore. A chamfer at the pad-side groove edge makes room for the seal to deform. As the piston advances towards the rotor, the inner surface of the seal grips the piston and flexes into that relief. On release, the seal recovers its shape and retracts the piston slightly. The dashed outline marks the relaxed shape. The groove profile is schematic, not a manufacturer-specific design.
Seal relaxed · relief space visible

Schematic groove geometry. Motion and clearances are exaggerated; actual seal and groove profiles vary.