What was found
IN CRUISE FLIGHT AT FL 350, WE HEARD A LOUD THUMP & BANG FROM REAR OF CABIN. EXPERIENCED A RAPID DECOMPRESSION AT THIS POINT, CABIN CLIMB OF 2000 FPM PLUS. FOLLOWED PROPER CHECKLIST PROCEDURES, MANUALLY DEPLOYED PASSENGER MASKS. TROUBLESHOOTING REVEALED A LOOSE RUBBER BOOT BETWEEN THE WATER SEPARATOR AND J-DUCT. THIS CAUSED A MAJOR LEAK AND PREVENTED THE AIRCRAFT FROM PRESSURIZING. TIGHTENED RUBBER BOOT CLAMP. PRESSURIZED AIRCRAFT, LEAK CHECKED CLAMP, LEAK CHECK GOOD IAW M/M CH 21-10-15.