What was found
AIRCRAFT FAILED TO PRESSURIZE ON DEPARTURE. DUMPED FUEL AND RETURNED. MAINTENANCE GROUND RAN AIRCRAFT, CARRIED OUT PRESSURIZATION CHECKS, NO FAULTS FOUND. AIRCRAFT PRESSURIZED NORMALLY TO MAX DIFF SUBSEQUENT FLIGHTS COMPLETED WITH NO FURTHER PROBLEMS SUSPECT GROUND AIR CONNECTOR CHECK VALVE MAY HAVE BEEN FROZEN IN OPEN POSITION AS AIRCRAFT HAD BEEN CONNECTED TO GROUND HEATER FOR EXTENDED WEEKEND LAYOVER.