What is the reason for the brakes becoming soft after changing the brake fluid?

[Etta]

After changing the brake fluid, the brakes become soft because the air was not completely purged during the replacement process. It is necessary to rechange the brake fluid and bleed the air. If the brake fluid has not been changed for a long time, corrosion inside the brake pump may also cause the brakes to become soft, requiring the replacement of the brake pump. There are three key points to note when changing brake fluid: 1. Do not allow air to mix into the brake fluid: Changing brake fluid requires coordination between two people. To prevent accidental air entry, one person should drain the fluid and observe, while the other repeatedly presses the brake pedal. New fluid must be replenished promptly and continuously. 2. Tighten the drain bolt to prevent leaks: Brake fluid is corrosive and can affect other components of the car. 3. Do not allow impurities or dust to mix into the brake fluid: Brake fluid, also known as hydraulic brake fluid or brake oil, is made from refined diesel distillate with thickeners and antioxidants. There are three types: mineral oil brake fluid, alcohol-based brake fluid, and synthetic brake fluid.