AN AIRBUS 320 IN CRUISE AT FL310 DSNDED BELOW ASSIGNED ALT WHILE TROUBLESHOOTING AN AUTOBRAKE PROB. MAINT REQUEST TO EXTEND GEAR EXCEEDED FOM LIMITS.

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AFTER LEVELOFF AT FL310; FO NOTICED THAT AUTOBRAKE MAX (REJECTED TKOF) LIGHT STILL ILLUMINATED. SHOULD HAVE GONE OUT AUTOMATICALLY AFTER AIRBORNE FOR 10 SECONDS. HAD FO CONTACT OUR MAINT CTL FOR ADVISEMENT. MAINT WANTED US TO CYCLE LNDG GEAR (EXTEND AND RETRACT) TO SEE IF LIGHT WOULD EXTINGUISH. I SLOWED ACFT TO 220 KIAS WHICH IS GOOD FOR VLO EXTENSION AND RETRACTION. WITH AUTOPLT ON; I HAD FO EXTEND LNDG GEAR. AS GEAR DOORS OPENED AND GEAR EXTENDED; ACFT BALLOONED AND CLBED TO APPROX FL311.5. AUTOPLT RETURNED ACFT TO FL310. I THEN HAD FO RETRACT LNDG GEAR AND AS GEAR DOORS OPENED AND GEAR CAME UP; THE ACFT BEGAN DSCNT. I MONITORED AS AUTOPLT CORRECTED UP AND AUTOTHRUST APPLIED PWR. HOWEVER; THE DSCNT WAS NOT ARRESTED AND REVERSED UNTIL APPROX FL306.5. BEFORE WE DID THIS PROC; WE ADVISED ZLA WE WOULD BE SLOWING AND CYCLING LNDG GEAR. THIS PROC DID NOT TAKE CARE OF OUR LIGHT PROB. HOWEVER; WE DID DETERMINE THAT AUTOBRAKES WERE OFF BY ECAM (WHEELS DISPLAY). WHAT HAD MY FO AND I BAFFLED IS WHY THE ACFT DSNDED 350 FT WHILE WE RETRACTED THE LNDG GEAR. SINCE THIS WAS MY FIRST TRIP OFF OF IOE AND MY FO BEING RELATIVELY NEW TO THE ACFT (4 MONTHS SINCE CHKING OUT); WE DECIDED TO LOOK IN THE ACFT FOM FOR CLUES. WE FOUND IT. MAX ALT FOR LNDG GEAR OP IS FL250. WHILE WE HAD COMPLIED WITH THE SPD FOR OPERATING THE GEAR; WE HAD BOTH FORGOTTEN THE OPERATING ALT.