What was found
CANNOT KEEP THE CORRECT CABIN PRESSURE. CAUSED CABIN ALTITUDE LIGHT AND HORN TO GO OFF. DESCENT BELOW 10000FT. 1 PSI DIFF PRESSURE. RT PACK ALREADY ON MEL. EMERGENCY DECLARED. COMPLETED CABIN PRESS LEAK TEST REF TASK 05-51-91-790-801. LEAK CHECK GOOD. ACCOMPLISH TASK 21-00-05-780-801. TEST GOOD AT CONFIDENCE HIGH WITH HIGH FLOW AND LOW FLOW. PHASE 1 TESTED WITHIN LIMITS. PHASE 2 HIGH AND LOW FLOW MODES TEST WITH LIMITS. PHASE 3 TEST GOOD. WITHIN LIMITS CABIN PSI STARTED AT 4 WITHIN 2 MINUTES DROP TO 3. REPLACED AFT PAX TRIM AIR VALVE REF AMM 21-61-21 TO CLEAR RT PACK MEL. OPS AND LEAK CHECK GOOD.