As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON CLIMBOUT AT APPROX 12500, HEARD THUMP AND IMM CABIN PRESS BUMP. LEVEL OFF AT 14000. RETURNED TO MKE. ON DESCENT FLOW LIGHT WAS ILLUM WHEN THROTTLES WERE RETARDED. FOUND OUTLET DUCT OF LEFT COOLING TURBINE OFF. RESECURED AND TORQUED PER DMM 21-51-01-2. LEAK CHECKED DUCT AND RAN A/C PACK PER 21-00-00-0. RAN BOTH PACKS WITH ENG RUNNING. VERIFIED BOTH PACKS, PRESS AIRCRAFT NORMAL.