What was found
(CAN) DURING DEPARTURE, THE ACFT WAS UNABLE TO PRESSURIZE. THE CREW RETURNED TO THE AIRPORT DUE TO NOT BEING ABLE TO CONTROL PRESSURIZATION AFTER TAKEOFF DURING THE CLIMB IN AUTO AND MANUAL PRESSURIZATION MODES. CREW REPORTED THE CABIN ALTITUDE WAS INCREASING WITH THE VERTICAL SPEED. UPON INITIAL INVESTIGATION BY MAINTENANCE AT THE GATE, THEY CONFIRMED THE AFT CARGO CANTED BLOW OUT PANEL WAS POPPED OUT. DURING TROUBLESHOOTING MAINTENANCE FOUND WATER IN THE CARGO DOOR SEAL SYS. MAINTENANCE REPLACED THE CARGO DOOR SEAL PRESSURIZATION DESICCANT FILTER, PN 8SC5028-001 IAW AMM 52-13-26 AND PURGED THE DOOR SEAL LINES WITH NITROGEN. FOUND DURING TROUBLESHOOTING THE CARGO DOOR SEAL CONTROL VALVE WAS FROZEN. MAINTENANCE REPLACED THE CONTROL VALVE, PN 4100S031-01 IAW AMM 52-31-31 AND CARRIED OUT ALL PRESSURIZATION RUNS IAW AMM 21-31-00 AND 21-00-00 WITH NO FAULTS NOTED. ACFT RETURNED TO SERVICE. ACFT AFH 2676.12, AFC 1935, CARGO DOOR SEAL CONTROL VALVE IS OEM. TSN 2676.12 HRS AND 1935 CYC.