As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BOS - FLT 6216 - DURING TAXI OUT, MAIN AND OUTBOARD BRAKE PRESSURE DROPPED TO ZERO. CREW STOPPED A/C ON TAXIWAY. A/C TOWED TO GATE. MAINTENANCE FOUND NR 4 BRAKE SPUN AND SWIVEL LINE BROKEN. BRAKE AND SWIVEL REPLACED. A/C RETURNED TO SERVICE. (M)