One problem related to brake master cylinder has been reported for the 2003 Lexus IS300. The most recently reported issues are listed below.
The brakes failed at 143,840 miles. The brakes failure occurred while driving the car on a busy city street. After inspection by a mechanic at firestone auto care. . It was determined that a noticeable internal leak was coming from power brake booster. It was also determined that the master cylinder and power brake booster must be replaced.