As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI-OUT THE PILOT NOTED THAT THE LEFT BRAKE LOST PRESSURE AS THE PLANE TURNED ONTO THE MAIN RUNWAY. THE ENGINE WAS SHUT DOWN AND THE AIRCRAFT WAS TOWED BACK TO THE RAMP. MAINTENANCE DETERMIND THE AIRCRAFT NEEDED A BRAKE MASTER CYLINDER.