The Air Force believes it has found the source of pilots’ breathing problems in its F-22 Raptors and has lifted restrictions on long-duration flights for deployments, aircraft deliveries and repositioning of the aircraft, the Pentagon said today.

The easing of flight restrictions could allow the Hawaii Air National Guard to receive the final six of its 20 Raptors — a delivery that had been delayed by the ongoing oxygen-supply problems.

The Air Force determined that a malfunctioning valve caused an upper part of a pressure suit used at high altitudes to remain inflated when it should have deflated, causing pressure on the chest and breathing problems, the Pentagon said.

Additionally, the Air Force removed a canister filter from the oxygen delivery system that may have restricted air flow, and is making other improvements to delivery hoses.

The Air Force kept the stealth jets flying in recent months as it continued to examine the cause of an increasing number of hypoxia incidents, or pilots not getting enough oxygen.

“With any aircraft — be it the F-22 or the F-16 (or) with a helicopter or a ground vehicle, we can never take the risk to zero,” Pentagon Press Secretary George Little said today. “But we have an obligation to our troops and our airmen to make whatever equipment they are using as safe as possible, and that’s what we think we’re doing here.”

A Hawaii Air National Guard F-22 Raptor pilot declared an “in-flight emergency” July 6 after experiencing momentary dizziness, but landed safely at Joint Base Pearl Harbor-Hickam.

It was the first reported case of hypoxia-like symptoms experienced by a Hawaii-based pilot, the Hawaii National Guard said.

The Air Force said the pilot experienced an in-flight oxygen system malfunction on a routine F-22 training sortie, and the problem was categorized as a physiological “cause known” event — rather than the series of problems that were unexplained until the Pentagon’s announcement today.