What was found
DFW/ANC AIR TURNBACK, AT FL 31 CABIN ALT WENT TO 10 WITH DIFFERENTIAL AT 6.8 AND UNABLE TO PREVENT CABIN FROM CLIMBING 50-100 FPM. AIRCRAFT DESCENDED TO FL 29.0 THEN FL 25.0. MAX DIFFERENTIAL THAT COULD BE HELD WAS 6.5 AND CABIN CONTINUED TO CLIMB 50-100 FPM. BOTH OUTFLOW VALVES WERE FULLY CLOSED IN BOTH AUTO AND MANUAL. NO FUEL DUMPED. FOUND SUBSTANTIAL AIR LEAK LEFT WHEEL WELL PNEUMATIC DUCT AT 2 CLAMPS. REPLACED SEALS AT BOTH CLAMPS IAW MM 36-11-01. PRESSURIZED SYSTEM AND OPS NORMAL. AS PRECAUTION CHECKED A/C FOR LEAKS DURING PRESSURIZATION. NONE FOUND. ALL CHECKS NORMAL. (M)