As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BOS - FLT2039 - ON CLIMB-OUT FROM BOS, EN ROUTE TO DCA, THE B SYSTEM HYDRAULIC FLUID BEGAN TO DECREASE AND THE B SYSTEM ENGINE DRIVEN PUMP LOW PRESSURE LIGHT AND MASTER CAUTION LIGHT ILLUMINATED. CREW DECLARED AN EMERGENCY AND RETURNED TO FIELD AT BOS. FLIGHT LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REMOVED AND REPLACED THE NR 2 ENGINE DRIVEN HYDRAULIC PUMP AND B SYSTEM PRESSURE FILTER. OPERATIONAL CHECK AND LEAK CHECK GOOD.