What was found
SUPLEMENTAL WE WERE CLIMBING THROUGH FL180, WHEN WE PERFORMED ANTI-ICE/PNEUMATIC TEST WE NOTICED CAB ALT WENT VERY RAPIDLY TO AMBER AROUND 6,000FT CAB/ALT AT THAT POINT WE STARTED THE EMERGENCY DESCENT AND DECLARE EMERGENCY, CABIN ALT WENT TO 11,8000FT WITH CABIN WARNING "RED" IN EICAS WHILE DESCENDING. C/A-DEFERRED AS PER MEL 21-19. AUTH# 010446DM-C M-PROCEDURES C/W. PERFORMED OPC OF PRESSURIZATION CONTROL SYS. IAW AMM 21-31-00-700-801-A & 21-31-00-700-801-A. OPC GOOD. NO DEFECTS NOTED. MEL RESTORED.