A G200 IN CRUISE AT FL390 MADE AN EMER DSCNT DUE TO CABIN PRESSURE INCREASING SLOWLY FROM GREEN INDICATION 8.8 PSI; TO AMBER 8.9; AND THEN TO RED AT 9.0-9.2 PSI. THEY WERE FINALLY ABLE TO CTL CABIN PRESSURE MANUALLY AT FL310.

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ACFT; PART 135 CHARTER WITH TWO PAX; IN IMC CONDITIONS; HAD BEEN AT FL390 FOR APPROX 30 MINS. TIME WAS XA:45 ON FRI-04. RADAR AND RADIO CONTACT WITH ZZZ CTR. AT THE ABOVE STATED TIME; I NOTICED THAT THERE WAS A PROB WITH OUR PRESSURIZATION SYSTEM AND WE COULD NOT CTL IT. THE PRESSURE DIFFERENTIAL WAS INTO THE RED ZONE; SO WE WENT TO THE CHKLIST. WHILE GOING THROUGH THE CHKLIST; THE PROB WAS GETTING WORSE. I; PNF; ASKED FOR AN IMMEDIATE DSCNT TO A LOWER ALT. NO RESPONSE FROM ATC. I ASKED AGAIN AND LOOKED UP AND SAW OUR ALT WAS AT FL387 AND DECREASING. WHILE ALL OF THIS WAS GOING ON; I WAS ABLE TO MANUALLY TAKE CTL OF THE PRESSURIZATION SYSTEM. ATC INFORMED US; AFTER THE PF EXECUTED AN EMER MANEUVER (WITH TFC IN SIGHT); TO DSCND TO A SAFER ALT. WE CTLED THE PRESSURIZATION SYSTEM; LEVELED OFF AT FL310 AND CONTINUED TO OUR DEST. WE CALLED ZZZ AT XB:30. THIS PROB WAS SUBTLE; ONLY INDICATION ON EICAS WAS A SMALL PRESSURIZATION DIFFERENTIAL NUMBER GOING FROM GREEN 8.8 PSI TO AMBER 8.9 PSI TO RED AT 9.0-9.2 PSI. HAD TO SEARCH THROUGH CHKLIST TO FIND CORRECT PAGE; EVEN THOUGH THERE WAS A RED INDICATION (ON PRESSURIZATION DIFFERENTIAL) THERE WAS NO ALERTING SIGNALS AND NO MASTER WARNING/MASTER CAUTION. THE MAINT FACILITY STATED AT 10 PSI DIFFERENTIAL; THERE WOULD BE STRUCTURAL FAILURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT MAINT REPLACED THE AUTOMATIC PRESSURE CTLR AND TESTED OK. WHEN THE RPTR WAS ASKED IF THE OVER PRESSURE RELIEF WAS REPLACED OR TESTED; THE RPTR ANSWERED; 'NO; BUT MIGHT BE A GOOD IDEA.'