A G4 FLC EXPERIENCED A PRESSURIZATION LOSS AFTER LEVELING AT FL430. THE NATURE OF THE PROB AND A REQUEST FOR LOWER ALT WERE COMMUNICATED TO ATC; BUT THE FLC DID NOT DECLARE AN EMER AND DID NOT RECEIVE LOWER ALTS IMMEDIATELY. THE CRITICAL CONDITION OF THE ACFT WAS EVENTUALLY UNFOLDED THROUGH CTLR'S INQUIRY AND AN EMER WAS DECLARED FOR THE FLC. FLT DIVERTED.

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ON A FLT FROM TEB TO ASE; IN A G4 ACFT; HAVING RECEIVED A DIRECT CLRNC TO THE BAE VOR; IN COM WITH ZOB; AND IN THE VICINITY OF THE DKK VOR; AT FL430; WE NOTICED THAT THE CABIN PRESSURIZATION SYS WAS MAINTAINING A CABIN ALT HIGHER THAN NORMAL AND A DIFFERENTIAL PRESSURE LOWER THAN NORMAL. WITHIN 5 MINS AFTER LEVELING AT FL430; WE OBSERVED THAT THE CABIN ALT WAS INCREASING 100-200 FPM WITH A RESULTANT DECREASE IN THE DIFFERENTIAL PRESSURE. AFTER ATTEMPTING UNSUCCESSFULLY TO REMEDY THIS SIT BY USE OF BACKUP SYS; AND WITH THE CABIN ALT APCHING 9000 FT; WE REQUESTED A LOWER ALT; DONNED OUR OXYGEN MASKS; AND DEPLOYED THE PAX OXYGEN MASKS AS A PRECAUTION. WHEN I REQUESTED A LOWER ALT; THE CTLR ADVISED THAT THE WINDS WERE MORE ADVERSE AT FL350. I RESPONDED THAT OUR CONSIDERATION IN REQUESTING A LOWER ALT WAS A PRESSURIZATION PROB. WHILE DSNDING FROM FL430 TO FL350; THE CABIN RATE OF CLB INCREASED; SO I REQUESTED 15000 FT. WHEN THE CTLR INDICATED THAT THERE WOULD BE SOME DELAY IN ISSUING A CLRNC TO 15000 FT; I CONVEYED THE URGENCY OF OUR REQUEST; AND RECEIVED A CLRNC TO 15000 FT AT ABOUT THE TIME WE WERE DSNDING THROUGH FL350. THE CTLR ADVISED OF TFC AT FL330; AND REQUESTED A RPT OUT OF FL310. WE NOTICED NO CONFLICTING TFC ON OUR TCASII DISPLAY; AND RECEIVED NO TCASII WARNINGS THROUGHOUT OUR DSCNT TO 15000 FT. DURING THIS DSCNT; THE CTLR REQUESTED AN EXPLANATION OF OUR REQUESTS AND ACTIONS. DUE TO THE HIGH WORKLOAD AT THE TIME; WE CHOSE TO DEFER EXPLANATIONS UNTIL A SAFE ALT WAS OBTAINED. AT SOME TIME AFTER REACHING 15000 FT; THE CTLR ASKED IF WE WERE DECLARING AN EMER; AND IF WE REQUIRED ANY ASSISTANCE. WE RESPONDED 'NEGATIVE' TO THE EMER; AND ASKED THE CTLR TO STANDBY FOR OUR REQUEST FOR REVISED CLRNC. AT SOME POINT SHORTLY AFTER THAT EXCHANGE; THE CTLR ASKED WHAT OUR PROB HAD BEEN TO HAVE REQUIRED A DSCNT TO 15000 FT. WE RESPONDED THAT WE HAD EXPERIENCED A 'PRESSURIZATION PROB;' BUT THAT WE WERE NOT IN NEED OF ANY ASSISTANCE. WE DID NOT HAVE TO MAKE ANY EVASIVE MANEUVER; NOR DID WE HEAR ATC TELL ANY OTHER ACFT TO MAKE ANY DEVS. THE CTLR NEVER REQUESTED THAT WE CONTACT THEM BY PHONE AFTER LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS ACFT WAS FLOWN AS PART OF AN OPERATING AGREEMENT BTWN THE MANUFACTURER AND THE RPTR'S COMPANY. THE ACFT HAS SINCE BEEN RETURNED TO THE MANUFACTURER AS A RESULT OF ALL OF THE MAINT PROBS THAT THE COMPANY HAS EXPERIENCED. FOLLOWING THIS INCIDENT; POST FLT MAINT DISCOVERED THAT THE ACFT HELD THE MAX 'LEAK RECORD' FOR PRESSURIZATION PROBS IN THE G4. RPTR STATES THAT MAX ALLOWABLE PRESSURIZATION LEAK IS SET AT 26 UNITS; AND THAT THIS ACFT WAS MEASURED AT 62 UNITS. THE HISTORY BEHIND THIS ACFT CONTRIBUTED TO ITS MECHANICAL STATE. IT HAD BEEN SITTING FOR OVER 2 YRS AND THE SEALS WERE ON THE VERGE OF FAILING. CABIN CTL APPEARED NORMAL AT FL180 AND THE MID 20'S; BUT WHEN THIS FLC TOOK THE ACFT TO FL430; THEY REALIZED THAT THE OUTFLOW VALVE HAD BEEN WORKING OVERTIME AT THE LOWER ALTS TO MAINTAIN CABIN ALT. THE SLOW CABIN CLB RATE DID NOT APPEAR TO BE AN EMER TO THE FLC. THE RPTR STATES THAT HE CONSIDERED THE FLT STATUS TO BE URGENT. THE CTLR DECLARED AN EMER FOR THE FLC. FAA FOLLOW-UP INVESTIGATION DID NOT REQUIRE ANY PAPERWORK FROM THE FLC; AND THE RPTR WAS INFORMED THAT THIS WAS A COMMON EXPERIENCE. RPTR WAS ALSO TOLD THAT AS A GENERAL RULE ACR FLCS WERE NOW TOLD TO DECLARE AN AMER ANY TIME THAT THE FAILURE OF A COMPONENT AFFECTS THE SAFETY OF FLT.