What was found
AC COULD NOT MAINTAIN PRESSURIZATION AND INCREASE IN CABIN ALTITUDE RESULTED IN OXYGEN MASK DROP AND AURAL WARNINGS, AS APPROPRIATE. AC DESCENDED TO FL220 AND CREW WAS ABLE TO MANUALLY RESTORE CABIN ALTITUDE TO NORMAL. AC CONTINUED FLT TO HOME BASE, NORMALLY. AT SUBMISSION OF M&D REPORT, MALFUNCTION HAS NOT BEEN DETERMINED. 8010-4 WILL BE SUBMITTED ON DISCOVERY CAUSE OF PROBLEM.