A valve structure for treating a patient suffering from obstructive sleep apnea is provided. The valve structure is connected to an air flow generator and connected to a mask that covers at least the nostrils of a patient. The valve structure includes an inlet pressure port attached to the air flow generator and an expiration valve that includes an expiratory membrane, a primary seat and a secondary seat. During inspiration the expiratory membrane forms a seal with the primary seat, and during expiration, the expiratory membrane forms a seal with the secondary seat.
