What was found
BRAKES FAILED MOMENTARILY WHILE PULLING UP TO AN ACTIVE RUNWAY, DURING FURTHER TAXING LEFT BRAKE LAGGED THEN CAUGHT SUDDENLY , TESTED FROM BOTH LEFT AND RIGHT SEATS ALSO NEEDED EXTRA POWER TO TAXI. MAINTENANCE TROUBLE SHOT TO A BAD PARKING BRAKE VALVE, REMOVED AND REPLACED VALVE OPS CHECKS GOOD, MAINTENANCE ALSO REPLACED THE LEFT BRAKE ASSY. TAXI CHECK WAS PREFORMED WITH NO DEFECTS NOTED.