What was found
THE RIGHT HYDRAULIC SYSTEM FLUID QUANTITY DROPPED TO ZERO DURING TAXI FOR TAKEOFF. COM PROCEDURES WERE COMPLIED WITH. DUE TO A POSSIBLE LOSS OF BRAKING ACTION THE AIRCRAFT WAS TOWED BACK TO THE GATE. MAINTENANCE FOUND THAT THE NR 4 POSITION UPPER BRAKE FUSE WAS BROKEN AND HAD COME APART. THE FUSE WAS REPLACED AND LEAK CHECKED. THE ELECTRIC HYDRAULIC PUMP, THE ENGINE DRIVEN HYDRAULIC PUMP, THE HYDRAULIC FLUID QUANTITY LIMITER, AND THE FILTERS FOR THE RIGHT HYDRAULIC SYSTEM WERE ALL REPLACED. ASSORTED HYDRAULIC LINES WERE ALSO REPLACED. FUNCTIONAL CHECKS WERE GOOD AND THE AIRCRAFT HAS RETURNED TO SERVICE.