What was found
FLT NR 6586 - IAD-ISP - DURING CRUISE, DECLARED AN EMERGENCY AND LANDED AT ISP DUE TO THE PRESSURIZATION DUMPING AT 15,000 FT, THE 'CABIN' CAPTION ILLUMINATED, AND THE CABIN OXYGEN MASKS DEPLOYED. THE A/C LANDED AT ISP WITH NO FURTHER INCIDENT. HOWEVER, THE OXYGEN SYSTEM WAS READING 0. MX OPENED DMI NR 42402 IAW MEL 21-14 ON THE PRESS SYSTEM, AND DMI NR 42403 IAW MEL 35-1 ON THE OXYGEN SYSTEM. MX LATER CLEARED DMI NR 42403 ON 12-22-95, BY CLEANING THE OUTFLOW VALVE AND ALSO SECURING LOOSE SENSE LINE. OPS CHECK GOOD. CLEARED DMI NR 42403 ON 12-22-95, BY REPLACING SEAT 3BC STOWAGE OXYGEN MASK AND STOWED REMAINING OXYGEN MASKS. OPS CHECK GOOD. A/C WAS RELEASED AND RETURNED TO SERV. REF VTZA96051.