ACFT PRESSURIZATION PROB. CABIN ALT EXCEEDED 12000 FT WITHOUT THE ANNUNCIATOR ILLUMINATING. FIRST INDICATION OF EXCESSIVE CABIN ALT WAS DEPLOYED OXYGEN MASKS. FLC REGAINED PRESSURIZATION CTL.

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MY FO AND I WERE RELOCATING AN AIRPLANE FROM OAK TO RDD. WE HAD A DEAD HEADING PLT AND A MECH WHO WAS COMMUTING HOME RIDING IN THE BACK DURING THE PART 91 FLT. AT 1/2 HR INTO THE FLT AT FL180 THE DEAD HEADING PLT CAME FORWARD AND INFORMED ME THAT THE OXYGEN MASK HAD DEPLOYED IN THE CABIN. I THEN NOTICED THE CABIN ALT TO BE 12000 FT. THE CABIN ANNUNCIATOR NEVER ILLUMINATED; EVEN THOUGH IT TESTED OK. BOTH FLOWS WERE AT 7. WHEN I INCREASED THE L FLOW THE CABIN STARTED TO PRESSURIZE. IT SPIKED THE CABIN RATE TO 1000-1500 FPM. I THEN TURNED THE CABIN RATE DOWN. THE PLT AND MECH PROCEEDED TO REPLACE THE MASK. ALL THE MASKS WERE REPLACED BY THE TIME WE LANDED. THIS HAS BEEN A COMMON EVENT WITH THIS ACFT. I DECIDED NOT TO WRITE IT UP AS A MAINT DISCREPANCY AND INFORMED THE CAPT WHO WAS GOING TO FLY IT NEXT ON A REVENUE FLT. THIS WAS A COMMON PRACTICE AT OUR AIRLINE TO KEEP THE PLANES FROM BEING GNDED AT OUT-STATIONS. THE AIRPLANE DID NOT HAVE ANY PROBS THAT DAY. AFTER FURTHER THOUGHT I DECIDED THAT IN THE FUTURE I WILL WRITE UP ALL MAINT PROBS SO RECURRING DISCREPANCIES WILL BE FIXED.