A LR36 LNDED ON A 5000 FT RWY AND DEPARTED THE END OF THE RWY WHEN UNABLE TO STOP. THE PLT TAXIED ONTO A SVC ROAD BUT WAS STOPPED BY LOCAL ATC FROM TAXIING TO HIS FBO.

Narrative

CHAIN OF EVENTS: THE PIC WAS ACTING AS PF AND I; THE SIC WAS ACTING AS PNF. AFTER TKOF; WE ENCOUNTERED A PRESSURIZATION PROB. WE REQUESTED AND WERE CLRED TO LEVELOFF AT 8000 FT WHILE WE WORKED ON THE SYS MALFUNCTION. WHEN IT WAS DETERMINED THAT THE ACFT WOULD NOT PRESSURIZE PROPERLY; THE DECISION WAS MADE TO DIVERT TO ZZZ. THIS DECISION WAS BASED ON CUSTOMER PREFERENCE. WE WERE HANDED OFF TO APCH AND ADVISED THEM OF THE SIT. WE WERE GIVEN VECTORS AND TOLD THAT RWY XXL WAS AVAILABLE. I REVIEWED THE ARPT DIAGRAM AND ADVISED THAT RWY XXL WAS THE LONGER RWY. THE PIC WAS CONCERNED WITH WHICH RWY WAS CLOSER TO THE FBO WHICH WE WERE PLANNING ON GOING TO. APCH CTL SAID THAT XYR WAS CLOSER TO THE FBO; BUT IT WAS THE SHORTER RWY. THE PIC SAID TO REQUEST XYR. I VOICED MY CONCERN AGAIN ABOUT RWY DISTANCE; NOTING THAT WE WERE NEAR OUR MAX LNDG WT. THE PIC SAID HE FELT THAT XYR WOULD BE ACCEPTABLE AND HAD ME REQUEST IT. WE WERE CLRED FOR THE VISUAL APCH TO RWY XYR AND SUBSEQUENTLY HANDED OFF TO TWR. TWR CLRED US TO LAND ON RWY XYR. ALL CHKLISTS WERE COMPLETED PRIOR TO LNDG (ACFT WAS IN NORMAL LNDG CONFIGN -- GEAR DOWN AND FULL FLAPS). TOUCHDOWN APPEARED TO ME TO BE AT OR NEAR THE TOUCHDOWN ZONE; WITH AIRSPD AT APPROX VREF. THE PIC WAS UNABLE TO STOP THE ACFT ON THE RWY REMAINING. WE DEPARTED THE END OF RWY XYR; CONTINUING RELATIVELY STRAIGHT OUT ON THE HARD GND. AS WE LEFT THE RWY SURFACE; I ADVISED TWR THAT WE WERE 'OFF THE END.' I THOUGHT WE WOULD COME TO A STOP; BUT THE PIC ADDED PWR AND MANEUVERED THE ACFT UP A SMALL HILL AND ONTO A SMALL PAVED SVC ROAD. THE PIC CONTINUED TO TAXI THE ACFT UNTIL TOLD BY TWR TO HOLD OUR POS. SUPPLEMENTAL INFO FROM ACN 794572: FROM PAST EXPERIENCE I FELT THIS WOULD BE OK EVEN THOUGH PART 135 LNDG DISTANCE WAS ABOUT 5200 FT AND PART 91 WAS CLOSER TO 4000 FT. EVERYTHING WAS NORMAL AT TOUCHDOWN AND ROLLOUT. WITH 2500 FT TO GO; I BEGAN THINKING WE WERE FAST AND BEGAN REALLY HARD BRAKING. IT WAS NOT ENOUGH. WE LEFT THE RWY AT ABOUT 20 KTS DOWN A SWALE. NOT HARD GND AND NOT TOO ROUGH; I ADDED PWR TO GO UP A HILL TO A SVC ROAD; ONTO THE ROAD AND BEGAN TAXIING WHEN STOPPED BY TWR. CHK PERFORMANCE MORE CLOSELY.