What was found
AFTER ACFT DEPARTED AND PASSING 18,000 FEET, THE CABIN ALTITUDE BEGAN INCREASING. BY 19000 FEET THE CREW RECEIVED A CONTINUOUS REPETITIVE CHIME MASTER WARNING ALERT. ATTEMPTS TO RECOVER THE PRESSURIZATION WERE UNSUCCESSFUL. THE CREW COMPLETED THEIR QRH PROCEDURES. DECLARED AN EMER & DID AN AIR TURN BACK. THE ACFT MADE AN UNEVENTFUL LANDING. TROUBLESHOT THE SYS & FOUND THE MAIN CARGO DOOR SEAL LEAKING. REPLACED THE SEAL IAW MM 72 JIC 52-31-40 RAI-10000. THE SYS CHECKED GOOD IAW 72 MM JIC 05-53-00 TST-10000.