What was found
AIRCRAFT PERFORMED A NORMAL TAKE OFF. CLIMBING THROUGH 11,500 FT, THE MASTER CAUTION WARNING LIGHT AND THE CABIN 10,000 FT SEGMENT LIGHT ILLUMINATED. CREW RAN THE NON-NORMAL CHECKLIST AND CHOSE TO HEAD BACK TO DEPARTURE. NO EMERGENCY WAS DECLARED. AIRCRAFT PERFORMED A NORMAL LANDING WITH NOR FURTHER INCIDENCE. THE DECISION WAS MADE TO MEL THE PRESSURIZATION SYSTEM AND HAVE THE AIRCRAFT RETURN UNPRESSURIZED. AFTER EXTENSIVE TROUBLESHOOTING IT WAS FOUND TO BE A PNEUMATIC RELAY THAT HAD FAILED. PNEUMATIC RELAY WAS REPLACED. THE AIRCRAFT PASSED MAINTENANCE MANUAL GROUND OPERATIONAL CHECKS AND WILL BE TEST FLOWN PRIOR TO RELEASE FROM MAINTENANCE.