What was found
AFTER & IN THE CLIMB OUT WE HAD THE RAPID CAIN DEPRESSURIZATION WARNING. DID ALL THE EMERGENCY CHECKLIST & STOPPED THE CLIMB AT 12000 CABIN ALT, ON THE THE MANUAL & CONTROL BUT COULD NOT CONTROL THE DESCEND UNTIL WE GOT TO 10,000'. RETURNED BACK & LANDED WITH AN OVERWEIGHT OF 45558 POUNDS. HAD A SMOOTH LANDING WITHOUT EXCEEDING 300 FEET PER MINUTE. OVERWEIGHT LANDING WITHIN LIMITS IAW AMM 5-50-25. DEFERRED PRESSURIZATION IAW MEL 21-21. INSTALLED NEW GND CONNECTION VLV IAW AMM 21-22-04, OPERATIONS CHECKED GOOD. MEL ITEM RESTORED.