What was found
AT FL 310 COULD NOT CONTROL CABIN PRESSURE, 250 FPM TO 10000, DESCENDED DOWN TO 12000 FT AND DIVERTED. PREVIOUS FLT A MEL WAS ISSUED FOR AFT CABIN TEMP & RT PACK FLOW CONTROL VALVE DE-ACTIVATED & LOCKED IN CLOSED POSITION. MEL ALSO REQUIRES FLOW SWITCH BE PLACED IN HIGH POSITION, AFTER DIVERSION SWITCH FOUND IN NORMAL. MX PERFORMED CABIN PRESSURIZATION CHECK USING PACKS 1 & 2 INDEPENDENTLY WITH FLOW SELECT SWITCH IN HIGH, COND 1, OPS NORMAL. RE-ACCOMPLISHED PROCEDURE USING BOTH PACKS TOGETHER & OPS NORMAL. PERFORMED BITE TEST OF CABIN PRESSURE CONTROLLERS 1 & 2, CHECKED GOOD. BITE TEST OF CFDS FAILED AFT CABIN TRIM AIR VALVE 13HK FOR AFT CABIN TEMP PROBLEM. R&R AFT CABIN TRIM AIR VALVE, OPS CHECK GOOD.