Well...I don't really know why, but it seems to be fixed. I reinstalled the cheap aftermarket new valve I bought off Ebay, and this time I removed the gas cap and cleared the code while the engine was running. That seems to have done the trick, at least for now. I read on a different thread about that trick, but it does not really makes sense to me.

I had also checked a different vacuum valve at the top of the engine and the reading was also 8.5V, so I am guessing that value is okay. Fingers crossed.