What was found
EXPERIENCED A LOSS OF CABIN PRESSURIZATION AT FL400. SELECTING BLEED AIR TO HP MODE, USING ALTITUDE SELECT AND MANUAL CONTROLLER SELECTION DID NOT CHANGE THE 300-400 FPM CABIN RATE OF CLIMB. THERE WERE NO UNUSUAL SOUNDS AND THE ANTI-ICE WAS OFF. OPERATOR REPORTS LOSS OF CABIN PRESSURE IN FLIGHT. COMPLIED WITH SB680-21-08. ALL PRESSURIZATION AND OPERATIONAL CHECKS NORMAL. PARTS INSTALLED DUCT COUPLINGS S3295-8-15