What was found
BWI - FLT 262 - AFTER TAKEOFF FROM BWI, CAPTAIN WAS UNABLE TOPRESSURIZE AIRCRAFT. FLIGHT RETURNED TO FIELD AT BWI. NO EMERGENCY WASDECLARED AND THE FLIGHT LANDED WITH NO FURTHER INCIDENT. MX CLEANED WATERDRAIN LINES ON BOTH PACKS AND LINES TO LEFT AND RIGHT STANDBY TEMPERATURECONTROL VALVES. PACK FLOW CHECKED NORMAL AT MIXING CHAMBER. ALSO, FOUNDLARGE AIR LEAK AT REAR PRESURE BULKHEAD. SEAL WAS FOUND OUT OF THE TRACKWHERE APU DUCT ENTERS THE PRESSURIZED AREA. REPAIRED SAME. ENGINE RUNACCOMPLISHED AT ALL POWER SETTINGS. DUCT PRESSURE, PACK OPERATION ANDPRESSURIZATION OK. (M)