AC690 PLT HAD A TRACK DEV IN ZAU CLASS A; AFTER THE PLT BECAME HYPOXIC; WHEN THE ACFT WAS UNABLE TO MAINTAIN PRESSURIZATION BECAUSE OF PRESSURE LEAKS.

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ON OCT/WED/03; AT APPROX XA00 HRS LCL; I WAS CONDUCTING A MAINT CHK FLT ON AN AERO COMMANDER. I FILED AN IFR FLT PLAN FROM EYE TO BVT TO EYE. DURING THE CLB WE WERE HANDED OFF TO ZAU AND CLRED DIRECT TO THE BVT VOR. WE WERE CHKING FOR PRESSURE LEAKS IN THE ACFT AS WELL AS OTHER ITEMS. WE HAD ADVISED ATC THAT WE WERE CONDUCTING A MAINT TEST FLT. WHILE WE WERE AT ALT (FL310) WE WERE GIVEN A VECTOR TO FOLLOW AND WE REMAINED ON THE HEADING FOR A LONG PERIOD OF TIME. DUE TO THE PRESSURE LEAKS IN THE ACFT; THE CREW BECAME SLIGHTLY HYPOXIC. WHILE WE WERE COMPLETING OUR MAINT CHKS; I LOOKED AT THE GPS AND NOTICED THAT WE WERE OFF COURSE. I MADE A LARGE TURN TO TRY AND CORRECT THE PROB. ATC THEN ASKED ME IF I HAD MADE A TURN AND I RESPONDED THAT I HAD. CORRECTIVE ACTIONS: IMPROVED COMS BTWN ATC AND FLT CREWS WHILE CONDUCTING MAINT FLT CHKS TO ENSURE THAT THE FLT IS PROCEEDING SMOOTHLY AND WITHOUT PROBS. CONTINUE TO EDUCATE PLTS AND FLT CREWS ON THE DANGERS OF HYPOXIA AND HOW IT CAN CAUSE DISORIENTATION.