What was found
PASSING THROUGH 25,000 FEET, CABIN ALTITUDE WAS 6,500 FEET AND CLIMBING AT 700 FEET. THE DIFFERENTIAL PRESSURE WAS 2.1 AT 29,000 FEET, CABIN WAS AT 6,500 FEET AND DIFFERENTIAL PRESSURE WAS 2.8, STANDBY NO HELP. CAPTAIN WENT ON OXYGEN BECAUSE NOT FEELING WELL AND FLIGHT RETURNED TO ORD. REPLACED CABIN PRESSURE INDICATOR. TESTED PRESSURIZATION SYSTEM AND CHECKED OK. TESTED ENGINE BLEED SYSTEM AND NO LEAKS WERE FOUND.