What was found
LOSS OF CABIN PRESSURE AT FL340. CABIN CLIMBED TO @ 14,000 DURING THE DESCENT WITH THE CABIN CONTINUING TO CLIMB @700 FPM. COMPLIED WITH QRH NO OBVIOUS FUSELAGE LEAKS NOTED ON WALK AROUND. AUTOMATIC CABIN PRESSURE CONTROLLER 1 AND 2 BITE CHECK GOOD. OPERATIONAL CHECK OF OUTFLOW VALVE GOOD. TRIED PRESSURIZING A/C USING LEFT PACK (RIGHT PACK ON MEL). DIFFERENTIAL PRESSURE MAXED OUT AT 1.25 PSI. NOTED FORWARD CLAMP MISSING ON LEFT PACK OUTLET DUCT BOOT TO CABIN SUPPLY CHECK VALVE CAUSING SIGNIFICANT LEAK. CLAMP NIS AT BNA. PARTS ORDERED. REF 757 AMM 20-10-10-4 INSTALLED CLAMP (MPN BACC10JB204C236) ON BOOT FROM PACK OUTLET DUCT TO CABIN SUPPLY CHECK VALVE. A/C PRESSURIZED NORMAL (RAPID CLIMB TO 4 PSI) REF 757 AMM 21-31-00-5.