Final answer:
The purpose of residual pressure in a hydraulic brake system is to decrease applied brake pressure. Residual pressure keeps the brake components in contact and prevents air or moisture from entering the system.
Step-by-step explanation:
The purpose of residual pressure in a hydraulic brake system is to decrease applied brake pressure. When the driver releases the brake pedal, residual pressure helps to keep the brake shoes or pads in contact with the brake drum or rotor. This ensures that there is no excessive play or clearance between the brake components, and helps to reduce the time required for the brakes to engage when the pedal is pressed again. By maintaining a small amount of pressure in the system, residual pressure prevents any air or moisture from entering the hydraulic system, which could cause brake failure or corrosion over time.