What was found
ON DEPARTURE THE FLIGHT CREW EXPERIENCED A LT PACK HIGH PRESSURE CAUTION MESSAGE WITH RT PACK ALREADY ON MEL. THEY ACCOMPLISHED THE QRH, COMMUNICATED WITH DISPATCH, ADVISED ATC OF THE PROBLEM, REQUESTED DELAY VECTORS TO BURN OFF FUEL AND DECLARED AN EMERGENCY. THE CREW ADVISED THE FLIGHT ATTENDANT AND PASSENGERS OF THE SITUATION. THE FLIGHT RETURNED TO KHPN FOR A NORMAL APPROACH, LANDING, AND TAXI. NO ADVERSE PASSENGER REACTION TO THE EVENT WAS OBSERVED BY CREW MEMBERS DURING DEPLANING. AFTER TRANSFERING BLEED PER APPX B MEL HANDBOOK, UPON COMPLETING AND INCREASING POWER RECEIVED LT PACK OVER PRESS CAUTION MSG. C/W QRH MSG REMAINED, CONTINUE FROM 317082/4 A/C LANDED UNEVENTFULLY AT NORMAL LANDING WT OVER PRESS CONDICTION OCCURED AT APPROX 70%-76% N1.