USAF has discovered root cause of Raptor's oxygen problems

the Combat Edge upper-pressure garment's breathing regulator/anti-g (BRAG) valve is the main culprit along with the hose and connections to the aircraft. They're hoping to have the pilot's gear modified by September...

http://www.flightglobal.com/blogs/the-dewline/2012/07/usaf-has-discovered-root-cause.html

http://www.flightglobal.com/news/articles/usaf-pinpoints-root-cause-of-f-22-raptors-oxygen-woes-374690/