What was found
UPON LEVELING AT FL360, "CAB ALT" WARNING HORN SOUNDED. CAB ALT WAS 10,000' & CLIMBING. ACCOMPLISHED PHASE 1'S, RAN QRH, & WAS UNABLE TO REMEDY LACK OF PRESSURIZATION. ELECTED TO RETURN. CREW REPORTED DURING DEBRIEF THAT THEY COULD NOT CONTROL THE RT PACK, COOLING DOORS WORKED BUT TEMP REGULATION AND THAT IT APPEARED THE RT PACK WAS NOT ON. CREW REPORTED OUTFLOW VALVE WAS CLOSED, HOWEVER THEY DID NOT TRY MANUAL PRESSURE, STAYED IN AUTO PRESSURE. RT ACM OIL LEVEL TOP OF SIGHT GLASS REF AMM 12-23-001-2-0. PERFORMED OPS CHECK OF BOTH PACKS FOR 1 HR, OPS NORMAL. TEMP CHANGES AS COMMANDED. FULL COLD DUCT TEMP IS STEADY AT APPRX 40 DEGREES AND WATER IS STEADILY DRAINING OVERBOARD, IAW AMM 21-55-000-0-0. VISUALLY VERIFIED OUTFLOW VALVE FULLY CLOSES AND MATCHES C/P INDICATIONS. PERFORMED OPS CHECK OF AUTO, STANBY AND MANUAL AC/DC MODES IAW AMM 21-31-000-2.