B757 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 17250; Flight Crew Type 8000
Human-Machine Interface
Aircraft Equipment Problem Critical
General Declared Emergency; General Maintenance Action
Aircraft
During the descent we noticed the left duct pressure was continuously fluctuating. We made a Maintenance write-up and Maintenance locked out high stage system per the MEL. The left engine high pressure shut off (HPSOV) now was inoperative and secured closed. I accepted the aircraft for our next short flight up the coast. During our climb the LT ENG BLEED OFF light illuminated on the overhead panel and the left duct pressure dropped to 9 PSI. No other non-normal indications were observed. After we were in cruise at FL380; however; as I was communicating with Maintenance on ACARS about the problem the Cabin Alt; Master and Aural Warnings all illuminated. We put our oxygen masks on; requested and were given clearance for an immediate descent. We accomplished the Cabin Altitude Checklist from the QRC. Once at an altitude of 10;000 FT we evaluated our options and briefed the flight attendants and Passengers. They reported that; other than being a little startled with the rapid descent and slight ear discomfort; everyone was OK. The cabin altitude did not exceed approximately 12;500 and the passenger oxygen masks did/were not deployed. We determined we were able to proceed to our destination. I contacted Dispatch and Maintenance on #2 VHF radio and all agreed we were good to proceed to continue. Dispatch sent a new flight plan with the new burn and time. I requested Customer Service meet the aircraft in case any passengers may have had any concerns. The next day; Maintenance found the PRSOV Manual Override shaft broken and replaced it along with the regulator.
A B757 crew suffered pneumatic bleed valve problems on two successive flight segments culminating in a loss of pressurization and an emergency descent to 10;000 FT. Passenger masks did not deploy and they were able to complete the flight to destination.