What was found
EN ROUTE FROM PHL-MSP, THE CREW REPORTED VIA ACARS THAT THEY HAD AN AFT CARGO COMPARTMENT FIRE BELL, BUT THE INDICATION WENT OUT BEFORE THE FIRE PROCEDURE COULD BE INITIATED. ALL WAS OK, SO THE FLIGHT PROCEEDED. HOWEVER, AS THE FLIGHT BEGAN DESCENT INTO MSP, THE INDICATION RETURNED. CAPTAIN FIRED THE BOTTLES AND DECLARED AN EMERGENCY. FLIGHT LANDED WITHOUT FURTHER INCIDENT AND TAXIED TO THE GATE. AT THE TIME OF THE INCIDENT, THE AFT CARGO COMPARTMENT SMOKE DETECTION WAS ON MEL 26-19B. MAINTENANCE REMOVED AND REPLACED THE CARGO FIRE EXTINGUISHING CENTRAL CONTROL UNIT, BOTH FIRE BOTTLES, P/N 34200409-1, METERING VALVE ASSEMBLY P/N 52000857-1, DIVERTER VALVE ASSEMBLY P/N 52000859-2, AND UNIT P/N 100-0649-04,