A LEAR35 CLBING THROUGH FL270 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURIZATION CTL.

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FLT DEPARTED ZZZ AT XA09 ENRTE TO ZZZ1 WITH 2 PAX. ONCE ON ZZZ2 CTR'S FREQ; WE WERE CLRED TO FL290. WHILE CLBING THROUGH FL270; THE 'CABIN ALT' ANNUNCIATOR ILLUMINATED. WE OBSERVED THE CABIN ALT AT 9000 FT AND CABIN VSI AT JUST OVER 2000 FPM. THE PIC LEVELED OFF AND CALLED FOR THE 'CABIN ALT' CHKLIST. WE BOTH DONNED OUR OXYGEN; AND I COMPLETED THAT CHKLIST. WHILE I COMPLETED THE CHKLIST; THE PIC BEGAN TRYING TO USE THE MANUAL CABIN CTLR TO STABILIZE THE CABIN ALT. THE CABIN CONTINUED TO CLB; SO THE PIC INITIATED AN EMER DSCNT. THE EMER BLEEDS (PRESSURIZATION) THEN ACTIVATED. I TOLD ARTCC OF OUR EMER DSCNT; AND WE WERE INSTRUCTED TO TURN L 30 DEGS; AND DSND TO 11000 FT. WE DIALED '7700' INTO THE XPONDER. THE PIC DECIDED TO DSND BELOW 9000 FT; AND WHILE PASSING 11500 FT I INFORMED ATC OF OUR INTENTIONS. WHILE DSNDING THROUGH APPROX 15000 FT; THE CABIN BEGAN TO PRESSURIZE WITH THE EMER PRESSURIZATION. ALSO; THE MANUAL CABIN CTL BEGAN TO WORK NORMALLY. AT 9000 FT; WE WERE CLRED DIRECT TO ZZZ AND HANDED OFF TO ZZZ3 TRACON. TRACON GAVE US A NON EMER SQUAWK. THE CABIN STABILIZED AROUND 6000 FT CABIN ALT; AND REMAINED THERE UNTIL FINAL DSCNT INTO ZZZ; AT WHICH TIME THE BLEED AIR WAS SWITCHED 'OFF.' THE PAX WERE NOTIFIED OF THE MALFUNCTION DURING THE EMER DSCNT AFTER MY PLT DUTIES WERE UNDER CTL. THEY SEEMED RELATIVELY CALM AND UNDERSTANDING; ALTHOUGH THEY COMPLAINED OF EAR BLOCK. THE MAJOR FACTOR AFFECTING OUR PLT PERFORMANCE WAS ANXIETY. THERE WAS A REAL SENSE OF URGENCY FOR US TO COMPLETE TASKS AND GET LOWER FOR FEAR OF A COMPLETE LOSS OF PRESSURE; WHICH MAY HAVE AFFECTED OUR PERCEPTION OF THE SIT. THE FAA FSDO WAS NOTIFIED IN ACCORDANCE WITH FAR 135.19(C). MAINT WAS PERFORMED PRIOR TO THIS FLT DUE TO A WRITE-UP OF A MINOR PRESSURIZATION ISSUE. THE 'FLOW CONTROL VALVE' WAS REPLACED PER THE MAINT LOG; AND SEEMS A LIKELY CAUSE. IT SEEMED TO BE A PROB WITH THE PNEUMATICS RATHER THAN A CABIN LEAK OR OUTFLOW VALVE MALFUNCTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT HE RETURNED TO THE FIELD WHERE A MANUFACTURER'S SVC CTR WAS LOCATED AND THE PROB WAS QUICKLY RESOLVED. THE OVER PRESSURE RELIEF VALVE IN THE REAR BULKHEAD WAS FOUND TO HAVE BI-METALLIC CORROSION HOLDING THE VALVE SLIGHTLY OPEN. THE VALVE WAS REPLACED AND PRESSURIZATION SYS CHKED OK.