The brake pistons are located in the brake calliper. The hydraulic brake pressure, which is transmitted via the brake fluid, pushes the brake pistons out. The pistons in turn press against the brake pad, pressing it against the brake disk. If the pressure on the brake piston decreases, it moves back into the brake calliper, the so-called reset.
The elasticity of the rectangular ring is limited by its design and it only deforms to a certain extent. If the necessary travel of the brake piston exceeds this dimension (e.g. due to wear of the brake disk or brake pad), the pressure built up in the brake system exceeds the static friction between the rectangular ring and the brake piston and the latter slips a minimal distance. The rectangular ring is thus repositioned at a position slightly further back on the brake piston (new "zero point") and guides the brake piston back to this new point when the brake system is released. This ensures that the clearance** remains virtually the same even if the brake components wear (readjustment).
The further the brake piston is pushed out of the brake calliper due to wear on the pads or discs, the greater the residual volume of brake fluid that remains behind the piston when the brake is released. This additional fluid volume is taken from the expansion reservoir of the brake master cylinder, the level of which drops visibly as the brake components wear. The fluid level in the expansion reservoir therefore also serves as an indicator for the wear of brake discs and brake pads.
** The clearance is the distance between the brake pad and brake disk in the unactuated state. If the clearance is too large (e.g. due to worn or incorrect components), this results in long stopping distances and longer braking distances due to the delayed build-up of braking force.
> Solving problems with the brake - a list of problems and solutions
> The most common problems with disc brakes
> Cleaning and replacing brake parts for the best performance (The Loam Wolf)
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