B-737-300 RETURN AND LAND AFTER A REPEAT OF PREVIOUSLY RPTED ACFT EQUIP PROB MALFUNCTION IN THE PRESSURIZATION SYS.

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737-300 ARRIVED IAH WITH CREW RPTING THAT THEY FLEW 3 HRS AT 31000 FT THEN AFTER DSNDING THROUGH 24000 FT LOST CABIN PRESSURE CTL; AUTO FAIL; STANDBY AND WENT TO MANUAL CTL AND WERE ABLE TO CTL CABIN PRESSURE. NO EMER DECLARED OR MASKS DROPPED. THE ACFT HAD 2 PREVIOUS WRITE-UPS RELATED TO PRESSURIZATION WITHIN THE WK PRIOR TO THIS. MY SUPVR AND LEAD TECHNICIAN DIRECTED MYSELF AND CO-WORKER TO CHANGE THE PRESSURE CTLR AS IT HAD NOT BEEN CHANGED. BEFORE ACFT DEPARTED IAH I CALLED THE LEAD TECHNICIAN BACK TO ACFT AND DISCUSSED THIS SIT AND REPAIRS. I ALSO ASKED HIM IF HE FELT WE SHOULD PRESSURIZE ACFT ON GND; HE SAID NO. ACFT DEPARTED AND AIR RETURNED DUE TO PRESSURIZATION PROBS (CTL). THIS CREW GAVE MORE DETAILED INFO AS TO THE PROB; TO INCLUDE DUCT PRESSURE L AND R PACKS. DUE TO 20 PSI LOWER ON L PACK; WE FOUND A LEAK AT THE L FLOW CTL VALVE. AFTER THE END OF MY SHIFT; REPLACED AND REPAIRED L FLOW CTL VALVE AND CLAMP. THE CREW ALSO RPTED A LEAK AT AFT CARGO DOOR SEAL; THAT WAS REPLACED ALSO. NO FURTHER RPTS AS OF THIS DATE. THE CTLR WAS CHANGED IN ACCORDANCE WITH BOEING MAINT MANUAL AND DOES NOT CALL FOR PRESSURIZING THE ACFT. HAD THE INBOUND CREW NOTICED AND RPTED DUCT PRESSURES; I FEEL CERTAIN THIS ACFT WOULD NOT HAVE AIR RETURNED.