Why does the brake pedal become increasingly stiff after the engine is turned off?

[VonBentley]

After the engine is turned off, negative pressure is no longer generated, and the remaining negative pressure is released after two brake applications, making it more difficult to press the pedal. Below is a relevant introduction about car brakes: 1. Introduction to brakes: Brakes, also known as stopping devices, refer to the action of stopping or reducing the speed of moving locomotives, vehicles, other transportation tools, or machinery. The general principle of braking is to fix a wheel or disc on the high-speed shaft of the machine and install corresponding brake shoes, bands, or discs on the machine base, which generate braking torque under external force. The braking device is a mechanical braking device that can slow down the speed, also known as a decelerator. 2. Classification of brakes: Drum brakes, advantages of drum brakes, disadvantages of drum brakes, disc brakes, etc.