What was found
FLT 6121 - BOS-LGA - NR 3 BRAKE OVERHEATED DURING TAXI-OUT. RETURNED TO GATE. AIRCRAFT WAS DIFFICULT TO PUSHBACK. TOOL EXCESSIVE POWER TO TAXI AND PULLED TO RIGHT. AS SOON AS POWER WAS REMOVED ON TAXI AIRCRAFT CAME TO A STOP ON ITS OWN ACCORD. FOUND NR 3 BRAKE DEBOOST VALVE BYPASSING CAUSING BRAKE TO LOCKUP. REPLACED BRAKE PER MM 32-71-32, NR 3 TIRE PER MM 32-45-1. NR 3 DEBOOSTER VALVE PER MM 32-41-21. BLED NR 3 BRAKE. REPLCED NR 4 TIRE PER MM 32-45-1. (M)