Fault Phenomenon: Schwing S500 brakes function normally when cold. After long-distance driving (operating temperature), the brake pedal feels soft, braking distance increases, and eventually fails.

Troubleshooting Process:
– Checked brake fluid level: level normal, no leaks.
– Checked brake master cylinder: piston cup not damaged, return normal.
– Checked brake wheel cylinders: no oil leaks, piston moves freely.
– Measured brake fluid temperature at operating temperature: found brake fluid boiling point too low, causing vapor lock.

Root Cause: The brake fluid has absorbed water and deteriorated, causing vapor lock at high temperatures leading to brake failure.

Required Parts: Brake fluid and brake system bleeding service.