AFTER A NO CABIN PRESSURE SIT IS REALIZED; THE PIC OF AN HS25-400 OVERSHOOTS HIS ASSIGNED ALT DURING A RAPID; EMER DSCNT; WITH SPD BRAKES OUT.

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DURING A NORMAL CLB TO ASSIGNED ALT OF FL230 AND COMING OUT OF FL200 BOTH PLTS CONFIRMED THE CABIN WAS NOT PRESSURIZING. NOTIFIED ZAU WE WERE EXPERIENCING PRESSURIZATION PROBS AND WE NEEDED TO DSND IMMEDIATELY AND RETURN TO ROCKFORD OUR BASE OF OPS. WE WERE CLRED; 'DIRECT ROCKFORD; DSND TO MAINTAIN FL190.' I TOLD THE FO TO TELL THEM WE NEEDED LOWER THAN FL190; WHICH HE DID. WE WERE THEN CLRED TO MAINTAIN 17000 FT. DURING THIS CONVERSATION WITH ZAU; THE FO AND MYSELF WERE BUSY WITH DONNING OXYGEN MASKS; COMMUNICATING; AND EXECUTING AN EMER DSCNT TO THE ASSIGNED ATL AND MANEUVERING THE AIRPLANE 180 DEGS TO DIRECT ROCKFORD. THE HIGHEST ALT I RECALL SEEING ON THE ALTIMETER WAS FL204 WHEN WE STARTED DOWN. IN A HAWKER 400 WITH PWR TO IDLE AND SPD BRAKES FULLY DEPLOYED AND AIRSPD TO THE BARBER POLE WHICH WAS 300 KTS; IT WILL NOT TAKE VERY LONG TO COME DOWN 3000 FT TO OUR ASSIGNED ALT OF 17000 FT. KEEPING IN MIND; DURING OUR CLB THE PRESSURE VESSEL NEVER PRESSURIZED. MY CO-PLT AND MYSELF WERE BOTH EXPERIENCING HYPOXIA AND THAT UNEASY; LIGHTHEADED FEELING FROM LACK OF OXYGEN. (AS WE DISCUSSED THIS SIT LATER; AT THIS POINT OF EXPERIENCING THAT 'NOT FEELING WELL' WE'VE CONCLUDED THAT EACH CREW MEMBER SHOULD COMMUNICATING THIS 'FEELING' TO THE OTHER CREW MEMBER.) DURING THE EMER DSCNT PROC AND NOW ON OXYGEN; BUT NOT FULLY RECOVERED 100%; I DSNDED THROUGH 17000 FT AND ARRESTED THE DSCNT AT 15000 FT. CTR ASKED IF WE HAD CONTINUED ON DOWN TO 15000 FT. WE ANSWERED WE HAD AND THAT WE 'NEEDED' TO GET DOWN. AT THIS TIME; ZAU HANDED US OFF TO PROBABLY ANOTHER CTR FREQ (I REALLY CAN'T RECALL AT THIS TIME) AND EVENTUALLY TO ROCKFORD APCH. ONCE IN ROCKFORD APCH CTL'S AIRSPACE WE ADVISED THEM WE NEEDED TO BURN FUEL OFF BEFORE ATTEMPTING A LNDG. THEY VECTORED US TO THE SW QUADRANT TO REMAIN WITHIN 15 MI OF THE ROCKFORD VOR AND TO DSND TO 4000 FT; WHICH WE DID. ONCE WE STARTED DSNDING OUT OF 15000 FT; WE WERE AT A MENTAL LEVEL TO TROUBLESHOOT THE SIT. COMPLETING THE CHKLIST; WE WERE DUMBFOUNDED AS TO WHY WE DID NOT PRESSURIZE. ONCE WE WERE WITH ROCKFORD APCH AND BROUGHT THEM UP TO SPD OF THE SIT; WE RECYCLED THE MAIN AIR VALVES AND REGAINED CTL OF THE PRESSURIZATION. THE CABIN PRESSURIZED AND WE REQUESTED 10000 FT WHICH WAS APPROVED AND WE TESTED THE ABILITY OF THE CTLR TO MAINTAIN THE CABIN ALT SELECTED WHICH WORKED FLAWLESSLY. WE WERE BOTH COMFORTABLE WITH THE PROB BEING SATISFIED AND CONTINUED ON THE ASSIGNED TRIP WITH 3 MORE TKOFS TO FOLLOW WITH NO FURTHER OCCURRENCES. AS I SAID EARLIER; AFTER THIS SIT; MY FO AND MYSELF DISCUSSED IT IN DEPTH AT VARIOUS TIMES THROUGHOUT THE DAY. OUR BIGGEST CONCLUSION IS: ONCE EITHER CREW MEMBER IS FEELING UNCOMFORTABLE OR ILL; THEY MUST RELAY THESE FEELINGS ON TO THE OTHER CREW MEMBER. I BELIEVE WE BOTH BEGAN EXPERIENCING THESE FEELINGS ABOUT THE SAME TIME AND WERE QUESTIONING OURSELVES WHAT WAS GOING ON. HAD WE SHARED THESE FEELINGS WITH EACH OTHER WE COULD HAVE CONCLUDED THERE WAS A PROB BEFORE WE HAD GOTTEN INTO THE HYPOXIA STATE OF CONFUSION/DENIAL/DO-NOTHING. HERE'S THE HUMAN SIDE WHICH WE BOTH HAVE DISCUSSED ALSO: I'M 22 YRS SENIOR TO MY FO. COULD I HAVE BEEN SAYING; I'M NOT GOING TO LET THIS YOUNGSTER KNOW I'M NOT FEELING ALL THAT WELL.' AND COULD HE HAVE BEEN SAYING JUST THE OPPOSITE; 'I'M NOT FEELING WELL AND I'M NOT GOING TO LET THIS OLD GUY KNOW.' HMMMMM. CONCLUSION: OXYGEN DEPRIVATION CAN COME ON VERY SLOWLY AND IT TAKES TIME TO REALIZE THERE'S SOMETHING WRONG AT WHICH TIME THE BRAIN IS MORE OXYGEN DEPRIVED AND ITS ABILITY TO ASSESS IS GREATLY HINDERED. AS IN TRAINING; WE ALL LEARN EMER DSCNTS FROM HIGHER ALTS AND AUTOMATICALLY ARE CLRED 12000 FT TO 10000 FT. AN EMER DSCNT DOWN ONLY 3000 FT HAPPENS VERY QUICKLY AND THEN TO HAVE THE BRAIN RECOVERING FROM OXYGEN DEPRIVATION AT THE SAME TIME ONLY HINDERS THE ABILITY TO REACT. I DO RECALL WHEN CLRED TO 17000 FT THAT WAS NOT VERY LOW AND I FELT WE NEEDED TO BE LOWER. OXYGEN DEPRIVATION IS NOT A COMFORTABLE FEELING AND ONCE EXPERIENCING IT IN THIS MANNER IS AGREAT TEACHER. FORTUNATELY; MY FO AND MYSELF ARE IN EXCELLENT HEALTH.