As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAXI OUT FROM THE LOADING RAMP THE RT BRAKE PEDAL WAS NOT EFFECTIVE. THE PILOT TAXIED BACK TO THE LOADING RAMP WITHOUT INCIDENT. MX FOUND THE RT BRAKE MASTER CYLINDER WAS NOT HOLDING PRESSURE. REPLACED THE RT BRAKE MASTER CYLINDER WITH A SERVICEABLE PART.