That is what I assumed, too. But, when I had the combi out on the bench, I turned it upside down, and stuck a ratchet extension in the hole and banged it with a big hammer until I deformed the nut in there. Nothing moved at all. so, I wondered if it moved downward, towards the road, back towards the hammer while upsidedown, to open instead. Because the pressure side will always be the exhaust side (except when pump is running), thus failing closed would be upward. quite the mystery.