What was found
@APPROX FL190 CABIN ALT ALERT SOUNDED IMMEDIATELY STARTED DECENT TO 10000 FT. CABIN MASKS AUTO DEPLOYED WHILE CREW DOWNING OXYGEN. COMPLETED DECENT W/ QRC & QRH PROCEDURES AND RETURNED TO ORIGIN JAX<[01] DURING INITIAL PACK CONFIDENCE CHECK FOUND GROUND AIR CONDITIONING CHECK VALVE DAMAGED REF TTG8Q0138LR3 FOR REMOVAL AND INSTALLATION. PERFORME3D CABIN PRESSURIZATION CONDITIONAL INSPECTION IAW AMM 05-51-81 REF TTG8Q0137MMM AND PERFORMED EXCESSIVE CABIN PRESSURE LEAKAGE CONDITIONAL INSPECTION IAW MM 05-51-91 REF TTG8Q137 MQ0.