G400 CAPTAIN REPORTS PRESSURIZATION PROBLEMS CAUSED BY A DOOR SEAL AT FL430 REQUIRING AN EMERGENCY DESCENT AND DIVERSION.

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WHILE IN CRUISE AT FL430 A SQUEAL SUDDENLY DEVELOPED IN THE COCKPIT. MY FLT MECH INVESTIGATED AND RPTED THAT WE HAD A LEAK IN THE MAIN CABIN DOOR SEAL. THE ACFT WAS HAVING NO TROUBLE MAINTAINING CABIN PRESSURE. WE WERE APCHING AN AREA OF WX AND DECIDED THAT AFTER DEVIATING S WE WOULD DSND TO LOWER THE CABIN DIFFERENTIAL PRESSURE AND CONTINUE. A FEW MINS LATER THERE WAS A SUDDEN INCREASE IN PITCH AND VOLUME OF THE SOUND. THE CABIN ALT BEGAN TO CLB AT AROUND 1500 FPM. THE OUTFLOW VALVE WAS FULLY CLOSED AND THE CABIN CONTINUED TO CLB. I DONNED MY OXYGEN MASK (THE FO WAS ALREADY WEARING HIS) AND CALLED FOR THE 'LOSS OF PRESSURIZATION' CHKLIST. I COMMANDED FO TO DECLARE AN EMER AND GET DIRECT ZZZ. CTR CLRED US TO 10000 FT AND DIRECT ZZZ. I INITIATED A R TURN AND BEGAN THE DSCNT IN ACCORDANCE WITH THE QRH. AT 10000 FT WE REMOVED OUR MASK AND GOT A HDOF TO ZZZ APCH. ZZZ GAVE US VECTORS FOR RWY XXR. I REQUESTED AND WAS GIVEN THE INBOUND COURSE AND LOC FREQ FOR THE RWY. WE ALSO REQUESTED THAT ATC NOTIFY UNITED STATES CUSTOMS OF OUR DIVERSION. WE COMPLETED A VISUAL APCH TO RWY XXR AND LANDED WITHOUT INCIDENT. WE TAXIED TO UNITED STATES CUSTOMS AND CLRED ABOUT AN HR LATER. THE FLT MECH NOTIFIED ME THAT BTWN FL200 AND FL250 THE CABIN BEGAN TO STABILIZE; PROBABLY DUE TO THE DECREASE IN DIFFERENTIAL PRESSURE. ON FURTHER INVESTIGATION HE DISCOVERED THAT THE DOOR SEAL HAD 'ROLLED' OUT OF ITS TRACK; MOST LIKELY WHEN WE CLOSED IT ON DEP.