I don't really know about the really old ones, but all the compressors I'm familiar with have the relief valve, as you mentioned. And I've seen that relief valve go bad and leak air but have never seen one fail the other direction and freeze up, although I suppose all things are possible.
One other thing to think about if you decide to use PVC. Gray schedule 40 conduit might be the stuff to use. The gray is UV resistant so it ought to last a little longer without becoming brittle if it exposed to sunlight.
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