What was found
DURING A PART 91 REPOSITIONING FLIGHT FOLLOWING MAINTENANCE AT KALN, WHILE CLIMBING THROUGH FL190, THE FLT CREW RECEIVED A FIRE EXT BOTL LOW ANNUNCIATION, INDICATING THAT ONE OR BOTH TAILCONE-MOUNTED ENGINE FIRE BOTTLES HAVE LOW PRESSURE, OR HAVE SELF-DISCHARGED VIA THEIR OVERPRESSURE RELIEF VALVES (AT APPROXIMATELY 210 DEGREES F). IAW COMPANY PROCEDURES (TREATING THIS AS A POTENTIAL TAILCONE/BAGGAGE AREA FIRE), CREW DECLARED AN EMERGENCY & RETURNED TO KALN. ON LANDING ARFF OBSERVED ACFT VIA FLIR & ADVISED THEY OBSERVED NO EXTERNAL SIGNS OF FIRE, & PIC INSPECTED THE INTERNAL TAILCONE/BAGGAGE AREAS & FOUND NO EVIDENCE OF FIRE. ACFT TAXIED TO MAINT FACILITY FOR TROUBLESHOOTING. MAINT DETERMINED THAT NEITHER FIRE EXTINGUISHER BOTTLE HAD DISCHARGED. FURTHER TROUBLESHOOTING IDENTIFIED AFT FIRE EXTINGR BOTTLE, WHICH HAD RECENTLY BEEN OVERHAULED, AS BEING UNDER-SERVICED. BOTTLE PRESSURE SUFFICIENTLY LOW THE PRESSURE SWITCH INDICATED A LOW-PRESSURE CONDITION AT AMBIENT TEMPS, BELOW APPROXIMATELY 85 °F, RESULTING IN THE FIRE EXT BOTL LOW WARNING. BOTH FIRE BOTTLES WERE R & R WITH NEW UNITS. TESTING COMPLETED SATISY, & THE ACFT RETURNED TO SERVICE.