What was found
AIRCRAFT WOULD NOT MAINTAIN PRESSURIZATION. CABIN CLIMBED AT 1500 FPM. CARGO HEAT OUTFLOW CLOSED, CABIN CLIMBED AT 500 FPM, AIR TURN BACK. ALL DOOR SEALS AND DUCTS CHECKED GOOD. NO LEAKS IN PRESSURIZATION CHECKS, CLEANED OUTFLOW VALVE AND FUNCTIONALLY CHECKED SYSTEM. SYSTEM OPS NORMAL PER MM 21-31-0. (M)