Narrative:

Aircraft was placed on MEL for left pack inoperative during overnight maintenance. Preflight; start; taxi-out; and takeoff were normal. On climb out; airflow from aircraft felt low and we observed right duct pressure 10 psi and left duct 50+ psi. There was no EICAS message and no lights were on except left pac; which was appropriate due to the MEL. As we continued to climb; the right duct pressure stayed at 10 psi. I assigned the first officer as pilot flying and told him to request FL200 while I contacted maintenance on ACARS.at this point I elected to cross bleed using the left engine bleed to supply the working right pack. Since this was the only thing that seemed to work and it was not the normal way to operate the system I told the first officer to advise ATC we would be returning to ZZZ. About this time; maintenance suggested we operate the pack with the cross bleed as we were already doing. We then declared an emergency; advised the flight attendants; dispatch; and the passengers. We held so we could burn down to max landing weight and then returned to the departure airport.while contract maintenance worked on the problem; dispatch called and asked if we would be willing to ferry the airplane back with the right pack operating from the left bleed through the isolation valve. We agreed and the ferry flight was uneventful.

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Original NASA ASRS Text

Title: A B757-200 returned to their departure airport when the right bleed failed to provide adequate pressure to operate the right pack. The flight had been dispatched with the left pack deferred inoperative.

Narrative: Aircraft was placed on MEL for left PACK inoperative during overnight maintenance. Preflight; start; taxi-out; and takeoff were normal. On climb out; airflow from aircraft felt low and we observed right duct pressure 10 PSI and left duct 50+ PSI. There was no EICAS message and no lights were on except LEFT PAC; which was appropriate due to the MEL. As we continued to climb; the right duct pressure stayed at 10 PSI. I assigned the First Officer as pilot flying and told him to request FL200 while I contacted Maintenance on ACARS.At this point I elected to cross bleed using the left engine bleed to supply the working right PACK. Since this was the only thing that seemed to work and it was not the normal way to operate the system I told the First Officer to advise ATC we would be returning to ZZZ. About this time; Maintenance suggested we operate the PACK with the cross bleed as we were already doing. We then declared an emergency; advised the flight attendants; Dispatch; and the passengers. We held so we could burn down to max landing weight and then returned to the departure airport.While Contract Maintenance worked on the problem; Dispatch called and asked if we would be willing to ferry the airplane back with the right PACK operating from the left bleed through the isolation valve. We agreed and the ferry flight was uneventful.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.