A B737-400'S OVER PRESSURIZATION RELIEF VALVE OPENED AT 2.0 PSID; CAUSING THE ACFT TO NOT PRESSURIZE PROPERLY. FLT RETURNED TO LAND.

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AFTER A NORMAL TKOF AND INITIAL CLB OUT; THE AFTER TKOF CHK WAS CALLED FOR AND ACCOMPLISHED. CLBING THROUGH 15000 FT; THE 'MASTER CAUTION' LIGHT CAME ON WITH AN 'AIR COND' ANNUNCIATOR. LOOKED UP AND SAW THE 'AUTO FAIL' AND THE GREEN 'STBY' LIGHT ILLUMINATED. NOTICING THAT THE CABIN WAS NOW OVER 7000 FT AND CLBING; SIGNIFYING THAT THE STANDBY SYSTEM WAS NOT CTLING THE CABIN; WE CALLED ATC AND TOLD THEM THAT WE WERE HAVING PRESSURIZATION PROBLEMS AND REQUESTED A DSCNT TO 10000 FT AND A TURN BACK TOWARDS ANC AND AWAY FROM HIGH TERRAIN. I THEN GOT OUT THE AUTO FAIL CHKLIST AND PROCEEDED TO ACCOMPLISH IT. THE COPLT WAS CTLING THE AIRPLANE AND GETTING SET UP FOR A RETURN TO ANC. THE HIGHEST THE CABIN CLBED TO WAS 10000 FT. PAX OXYGEN MASK DID NOT DEPLOY. AN EMER WAS NOT DECLARED. IN FOLLOWING THE STEPS TO THE AUTO FAIL CHKLIST I GOT TO STEP 7; OUTFLOW VALVE SWITCH -- AS REQUIRED AND NOTICED THAT I HAD SOME CTL OF THE CABIN; BUT I WAS NOT COMFORTABLE THAT I HAD FULL CTL BECAUSE THE CABIN WAS NOT DSNDING SLOWER THAN THE AIRPLANE WAS DSNDING. I RELAYED TO DISPATCH THAT I WOULD RATHER RETURN TO ANC. AN UNEVENTFUL APCH AND LNDG WAS DONE AT ANC. THERE WERE NO COMPLAINTS OR DIFFICULTIES RPTED FROM FLT ATTENDANTS OR PAX. AFTER A COUPLE OF HOURS TROUBLESHOOTING THE AUTO AND STANDBY PRESSURIZATION SYSTEMS AND FINDING NOTHING WRONG WITH EITHER SYSTEM CTLRS; THE ANC MECHANICS PRESSURIZED THE AIRPLANE ON THE GND USING ONE ENG. THE MECHANIC CAME TO US AND RPTED THAT WHEN THE PSID WAS PAST 2.0; PRESSURIZATION WAS LOST THROUGH THE GND AIR CONDITIONING DUCTING -- EVIDENTIALLY A 'FLAPPER' TYPE VALVE IN THE DUCTING OPENED WHEN THE PSID REACHED 2.0. HE SAID; 'THE DOOR JUST SHOT RIGHT OUT.'