What was found
FLIGHT CREW REPORTED, WOULD NOT HOLD RATED PRESSURE AT ALTITUDE WITH OUTFLOW VALVE CLOSED. CABIN CLIMB AT 5-600 FPM AND CABIN ALT AT 9,000 AT FL 29,000, CABIN CONTINUED CLIMBING UNTIL REACHING FL 21,000. MAINTENANCE PERSONNEL PERFORMED OPS CHECK ON PRESSURIZATION WITH APU AIR IAW MM 21-30-00. NO DEFECTS NOTED, ALSO PERFORMED EXTERNAL FOR LEAKS, NO MAJOR LEAKS FOUND, AIRCRAFT OK FOR EVALUATION FLIGHT. AIRBORNE SYSTEM TEST SATISFACTORY.