ACR MLG HAD PRESSURIZATION PROBLEM AT FL370 AND HAD TO DSND TO FL270 TO REGAIN CTL OF CABIN PRESSURE.

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I WAS FO ON AN MLG SCHEDULED FLT SFO/IND. I WAS THE PF. WE WERE CRUISING AT 37000 FT JUST E OF HUGO VOR WITH NORMAL CONVERSATION BTWN FLT CREW MEMBERS. SUDDENLY; THE ALT WARNING HORN WENT OFF (BEEPING TONE). AFTER A QUICK SCAN OF THE COCKPIT; WE REALIZED IT WAS A PRESSURIZATION PROBLEM. THE CABIN ALT WAS AT 10000 FT AND HOLDING; BUT THE PRESSURIZATION WAS SLOWLY BLEEDING OFF. I MOVED THE PRESSURIZATION CTLR TO STANDBY AND WE OBSERVED. THE PRESSURE HELD THE CABIN AT 10000 FT BUT DIFFERENTIAL KEPT DECREASING. THE CAPT MOVED THE CTLR TO MANUAL CTL. THE CABIN ALT STILL HELD AT 10000 FT BUT PRESSURE DIFFERENTIAL WAS STILL UNCTLABLE. CAPT CONTACTED DENVER CENTER AND ADVISED WE NEEDED AN IMMEDIATE LOWER ALT DUE PRESSURE LOSS. ATC ADVISED US TO TURN L 30 DEG. WE REITERATED WE NEED LOWER AND CENTER ADVISED US TO DSND TO 24000 FT. I COMMENCED A DSCNT ON AUTOPLT AT 4000 PLUS FPM. THE RATE OF CLB ON THE PRESSURIZATION PANEL RETURNED TO ZERO AND WE GAINED CTL OF THE CABIN MANUALLY AT 2700 FT. WE PULLED OUT THE CHKLIST AND FOLLOWED THE PROC. HOWEVER; WE DID NOT PULL MASKS BECAUSE THE CABIN REMAINED AT 10000 FT. WE LOWERED THE CABIN TO 8000-8500 FT. WE ADVISED CENTER THE CABIN WAS HOLDING AND ADVISED THE FLT ATTENDANTS WHAT HAD HAPPENED. A FLT ATTENDANT CAME TO THE FLT DECK AND WE DISCUSSED THE PROBLEM AND WENT OVER THE PROC JUST IN CASE THE PROBLEM RECURRED. THE CAPT AND I DISCUSSED OUR COURSE OF ACTION AND DECIDED TO PROCEED TO INDIANAPOLIS AT 27000 FT; MONITORING THE CABIN AND DEST AND ALTERNATE WX. THE FLT PROCEEDED TO DEST WITHOUT FURTHER PROBLEMS. MAINT AT INDIANAPOLIS LATER DETERMINED A MECHANICAL PROBLEM IN PRESSURIZATION SYS.