As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT TAXIED, AT HOLD SHORT LINE TO RUNWAY, THE LEFT BRAKE FAILED. PILOT APPLIED FULL RT BRAKE AND WAS ABLE TO REGAIN FULL LT BRAKE EFFECTIVENESS BY PUMPING THE BRAKE. ACFT RETURNED TO BLOCKS FOR TROUBLESHOOTING. MX TROUBLESHOT SYS TO LT BRAKE MASTER CYLINDER. LT BRAKE MASTER CYLINDER R & R IAW MM 32-40-00.