What was found
AIRCRAFT WAS GROUNDED: DURING CLIMB OUT LEFT DUCT PRESSURE WAS 5PSI AND RIGHT 35PSI. PRESSURIZATION WAS FLUCTUATING BETWEEN 500 AND 1000 IN CLIMB. TALKED TO MOC ALL AGREED WE SHOULD GO SINGLE PACK DESCENDED TO FL240 AND WENT TO SWITCH TO SINGLE PACK WHEN NOTICED R PACK WAS PEGGED AT 200 DECIDED NOT GO SINGLE PACK ON RIGHT AS LEFT DUCT HAD NO PRESSURE AND RIGHT PACK WAS AT 200. AIR RETURN TO MIAMI. AIRCRAFT WAS NOT OVERWIGHT AND LANDED WITHOUT ISSUE. MAINTENANCE REPLACED LEFT ENGINE HIGH STAGE REGULATOR IAW AMM 36-11-07.