What was found
PASSING THROUGH 20,000 FEET, PRESSURIZATION FLOW LIGHT ILLUMINATED. CABIN PRESS BEGAN TO CLIMB, AT MAX RATE, RT PACK 0 FLOW. QRH REMEDIES INEFFECTIVE TO RESTORE RT PACK & LT PACK SINGLE OPERATIONS WOULD NOT HOLD CABIN PRESS. FLIGHT RETURNED & LANDED WITHOUT FURTHER INCIDENT. ACFT REMOVED FROM SERVICE. REPLACED BOTH WATER SEPARATORS IAW AMM 21-20-02. OPERATIONALLY & LEAK CHECKED GOOD.