As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE CLIMBING OUT OF MKE, THE CREW NOTICED THAT THE CABIN ALTITUDE RATE WAS THE SAME AS THE AIRCRAFT. THE AIRCRAFT WAS LEVELED AT 13,000FT WITH A DIFFERENTIAL PRESSURE OF 1.9 IN AUTO MODE. THE FLIGHT DESCENDED TO 12,000FT WITH 8,000FT INDICATED FOR CABIN ALTITUDE IN MANUAL MODE. THE AIRCRAFT THEN RETURNED TO MKE AND LANDED WITHOUT INCIDENT. MAINTENANCE FOUND THE CONNECTION BETWEEN THE RAM AIR VALVE AND THE CONDENSER LEAKING. THE DUCT WAS RE-CONNECTED AND THE PRESSURIZATION SYSTEM OPERATED NORMALLY.