What was found
THE FLIGHT CREW REPORTED THAT DURING BRAKING THE COPILOTS LEFT BRAKE PEDAL FADED DURING OPERATION. THE MAINTENANCE TEAM TROUBLESHOT THE BAKE SYSTEM AND FOUND THE LEFT MIXING VALVE INTERNAL SPRING STUCK IN THE CLOSED POSITION CAUSING PRESSURE BUILD UP IN THE CO-PILOTS BRAKE SYSTEM WHICH LET TO THE LEFT CO-PILOTS MASTER BRAKE CYLINDER CHECK VALVE TO FAIL. MAINTENANCE TEAM REMOVED AND REPLACED THE LEFT CO-PILOTS MASTER CYLINDER AND THE LEFT BRAKE MIXING VALVE. OPS CHECKS WERE SATISFACTORY AND THE AIRCRAFT WAS RETURNED TO SERVICE.