As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER PUSH OUT WITH NR 1 ENG RUNNING & UPON SETTING THE PARKING BRAKE, THE CAPTAIN`S SIDE LT BRAKE PEDAL MADE A POPPING NOISE, LOST ALL BRAKE PRESS. THE FIRST OFFICER`S LT BRAKE PEDAL CONTINUED TO WORK. INSTALLED SPRING ON CAPTAIN'S LT PEDAL. OPERATIONS CHECKED GOOD IAW AMM 27-21-03.