What was found
DURING TAKE-OFF ROLL, THE BRAKES BEGAN TO SURGE, CAUSING THE AIRCRAFT TO VEER ON THE RUNWAY. AT THE SAME TIME, A BRAKE SYSTEM 1 FAULT ECAM MESSAGE GENERATED. THE TAKE-OFF WAS REJECTED AT 40 KNOTS. THE FLIGHT RETURNED TO THE GATE WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE NORMAL BRAKE SERVO VALVE, FIN 18GG. OPERATIONALLY CHECKS OK.