As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: DURING TAXI THE FLIGHT CREW REPORTED FOR A SECOND TIME THAT LIGHT BRAKE APPLICATION RESULTS IN MAXIMUM BRAKING AND HARD PULL TO THE LEFT. FLIGHT CREW REQUESTED TOW BACK TO THE GATE AND REPORTED THAT NO ISSUES DURING TOW WITH PARKING BRAKE RELEASED. MAINTENANCE FOUND THE #4 BRAKE SERVO VALVE (18GG) FAULTING AND REPLACED IT PER AMM 32-42-48. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED WITH NO PULLING NOTED DURING TAXI.