Malfunctioning pressurization system forced an HS-125 flight crew to complete their flight at a lower altitude.

Narrative

The aircraft failed to pressurize properly as discovered on the climb checklist; which in our plane is called for at FL180. Prior to doing the climb checklist ATC gave us a climb clearance to FL250 with an expedite through FL230. It was during the check pressurization checklist through about FL230 when the aircraft was in a 2000 FPM climb the checklist item called for a pressurization gauge check. That's when it was discovered that the aircraft was partially pressurizing but not fully as the cabin altitude was about 9000 FT. We ask for a lower altitude immediately to 11000 MSL from ATC and were granted step-downs to 11000 MSL. After all appropriate checklists were complied with no emergency was declared or needed to be declared. After a trouble shooting session by the non-flying pilot with maintenance and the Chief Pilot; the airplane was determined to be unable to be pressurized for flight originally planned at FL400. After a thorough review of options we made an educated decision that we could continue toward destination on the Victor airways. The fuel at destination with the lower altitude to be flown indicated our reserve minimum fuel limitation would be +200 extra pounds when all reroutes with winds and temps where factored in. Since it was a clear VMC day we decided to continue on to destination at 11;000 MSL along the Victor airways. The passenger was asked if he would like to continue on since the plane was able to do the trip or land at any airport nearby and wait for a recovery plane. We explained that the plane would have a higher fuel burn rate; but that we had more than adequate amount to make it to his destination. The flight proceed uneventfully from that point. We made as shallow of descents as possible to avoid noticeable pressure changes to the eardrums of crew and passenger. The passenger deplaned and thanked us for keeping him in the loop of information that we were processing. At no point along the flight was safety ever neglected as each option was analyzed and decided upon. A landing at any airport along the way would have had no difference in my opinion since it would require the same caution required for descending unpressurized.