oil leak in an S80

I have a 2002 S80 non-turbo that has developed an oil leak in an oil line going from the engine to the radiator. It is leaking from what appears to be a Shrader valve in that line. My mechanic replaced the PVC valve block on suggestions from Volvo and that fixed it for a while, but the leak is back, not as much, but still leaking. Does anyone have any idea what this valve is there for, and why it might be leaking? thanks