What is the reason for the Shimano rear brake piston not retracting?

[DeMiguel]

Disc brake sticking and not retracting may be caused by water entering the brake fluid, leading to rust on the wheel cylinder piston. Here is relevant information: 1. Front disc rear drum: Disc brakes are also called hydraulic brakes or oil brakes; drum brakes are a traditional braking method where the brake pads rub against the wheel drum to slow down. Generally, vehicles on the market use disc brakes for the front wheels and drum brakes for the rear wheels, which is referred to as the "front disc rear drum" configuration. 2. Difference: The biggest difference between drum brakes and disc brakes lies in the heat dissipation issue. During high-speed driving, drum brakes have poor heat dissipation, making them prone to heat accumulation, which can lead to thermal fade and weakened braking force. In contrast, disc brakes have better ventilation, dissipate heat quickly, and are less likely to experience thermal fade.