Narrative:

During cruise at FL340 with single pack ops (right pac meled) got cabin altitude warning with EICAS message and aural annunciation. Checked cabin altitude and it was slightly above 10;000 feet. Donned O2 masks and got out QRH; first officer (first officer) ran QRH procedures. Requested lower altitude from center. Got descent to FL320 initially due to traffic. During QRH first officer stated that we had lost control of the cabin altitude and requested descent to 10;000 feet. Received the descent and expedited descent to 10;000. Initially cabin altitude remained above 10;000; but eventually came down below 10;000 enough to remove the O2 masks. Cabin altitude stabilized at about 5;600 feet when at aircraft altitude of 10;000 feet. Requested routing to ZZZ and then sent ACARS message to [dispatch] explaining issue and requesting fuel numbers for continued flight at 10;000 feet to ZZZ. Received numbers and continued toward ZZZ. [Dispatch] contacted us and requested a return to base [rtb] to [departure airport] if our FMS showed we had sufficient fuel. We diverted the FMS and checked the fuel and were able to rtb to [departure airport]. [Dispatch] sent fuel numbers and weather for [departure airport] and ZZZ1 and ZZZ center cleared us back to [departure airport] direct at 10;000 feet. After handoff to [departure airport] approach control we made an uneventful visual landing at [departure airport].single pack operations (right pack deferred) and subsequent fluctuation and momentary loss of left engine bleed pressure resulting in loss of pressurization control of the aircraft.very involved maintenance history on the right pack and engine bleed issues led to deferral of the right pack when the issue appears to be related (to the pilots anyway) to the fluctuations and loss of the engine bleed pressure on both the left and right sides.

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Original NASA ASRS Text

Title: B757-200 pilot reported encountering pressurization problems due to right pack being MELed.

Narrative: During cruise at FL340 with single pack ops (R PAC MELed) got CABIN ALT Warning with EICAS message and aural annunciation. Checked cabin altitude and it was slightly above 10;000 feet. Donned O2 masks and got out QRH; First Officer (FO) ran QRH procedures. Requested lower altitude from Center. Got descent to FL320 initially due to traffic. During QRH FO stated that we had lost control of the cabin altitude and requested descent to 10;000 feet. Received the descent and expedited descent to 10;000. Initially cabin altitude remained above 10;000; but eventually came down below 10;000 enough to remove the O2 masks. Cabin altitude stabilized at about 5;600 feet when at aircraft altitude of 10;000 feet. Requested routing to ZZZ and then sent ACARS message to [dispatch] explaining issue and requesting fuel numbers for continued flight at 10;000 feet to ZZZ. Received numbers and continued toward ZZZ. [Dispatch] contacted us and requested a Return to Base [RTB] to [departure airport] if our FMS showed we had sufficient fuel. We diverted the FMS and checked the fuel and were able to RTB to [departure airport]. [Dispatch] sent fuel numbers and weather for [departure airport] and ZZZ1 and ZZZ Center cleared us back to [departure airport] direct at 10;000 feet. After handoff to [departure airport] approach control we made an uneventful visual landing at [departure airport].Single pack operations (R Pack deferred) and subsequent fluctuation and momentary loss of L engine bleed pressure resulting in loss of pressurization control of the aircraft.Very involved maintenance history on the R pack and engine bleed issues led to deferral of the R Pack when the issue appears to be related (to the pilots anyway) to the fluctuations and loss of the engine bleed pressure on both the L and R sides.

Data retrieved from NASA's ASRS site 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.