What was found
AFTER TAKEOFF IN OUR NORMAL CONFIGURATION CABIN STARTED UP A HIGHER THAN NORMAL RATE. AFTER CLOSING THE CABIN OUT FLOW VALVE AND TURNING ON THE RT PACK, CABIN RETURNED TO NORMAL UNTIL WE REACHED 26,000' AT WHICH TIME THE CABIN WAS CLIMBING AT 800'/MIN, SAME AS THE ACFT. RAN OUR LOSS OF PRESSURIZATION V LIST, STARTED TO DESCEND AND RETURNED TO DEPARTURE. SWITCHING TO STANDBY HAD NO EFFECT AND AT THAT POINT NOTICED THE OUTFLOW VALVE WAS ALREADY FULLY CLOSED. SWITCHED TO BOTH AC & DC MANUAL TO MAKE SURE THAT THE OUTFLOW VALVE WAS CLOSED. IT WAS FULLY CLOSED BUT THE CABIN CONTINUED TO CLIMB. AFTER STARTING OUR DESCENT, FINALLY CAUGHT THE CABIN AND EVENTUALLY THE CABIN BEGAN TO DESCEND NORMALLY WITH BOTH PACKS ON & OUTFLOW VALVE CLOSED. THE CABIN AT ITS HIGHER POINT REACHED 12,000'. THE WARNING HORN WORKED AS ADVERTISED. VERIFIED PRESSURIZED AREA FOR LEAKS, NONE NOTED. NO EXTERNAL DAMAGE TO ACFT FUSELAGE. LT PACK FOUND INTERMITTENT. FOUND FWD PACK S/O VALVE NOT RESPONDING TO SW COMMANDS. REPLACED SAME IAW AMM 21-55-071-4 ALL OK. IAW MEM REQUEST ALSO ACCOMPLISHED JRH TESTS "PNEUMATIC SYS TEST" REF 8-17 AND "PRESSURIZATION CONTROL SYS OPS TEST" REF 8-21 ALL NORMAL.