[Rick Erwin]

Hi Henri,

I think the valves you are looking for are located at the juncture of the sidewall and the ceiling, above the R4 and L4 windows. (That's counting the front windscreens as R1 and L1.

There is a valve for each wing root fresh air vent. If there is more pressure on the cabin side of the valve, the valve closes keeping the pressurized air in the cabin. Conversely, if the cabin is unpressurized, there is more pressure on the wing root side of the valve, which allows fresh air from the wing root to flow into the cabin.

Rick