What are the reasons for incomplete retraction of disc brake pistons?

[FelixLee]

Dirty brake fluid, stuck valves in the master cylinder, excessively small free play, slave cylinder pistons not returning to the center point, and failed return springs can all lead to incomplete retraction of disc brakes. Relevant daily maintenance information is as follows: Brake pad replacement: The braking performance of a vehicle is ultimately determined by the brake pads, so maintaining the brake pads in good condition is the most direct maintenance method for the braking system. Brake pads and discs have a long service life but must be replaced once worn to a certain extent. Under normal urban driving conditions, the service life is approximately 50,000 kilometers for brake discs and 30,000 kilometers for brake pads. Brake fluid: The key to brake fluid maintenance is ensuring it does not deteriorate, particularly preventing moisture ingress. Brake fluid should be replaced every 50,000 kilometers. If driving frequently in humid areas, the replacement interval should be appropriately shortened.