How do you send your data to WU, via Meteobridge?
Based on how I understaand it, MB automatically expects that you give it raw data from the station, i.e. the station pressure. Based on that it performs the conversion. So if you sent it adjusted values, then it applied the adjustment again, which is why you saw the incorrect values.
Now the question is how you update WU, if via MB, it should be ok. Obviously however, if your station updates WU directly, then it will now use the station pressure. In such case I would turn off the direct WU updates and set them up in MB control panel, that way it will be the MB that is sending the data and that should be correct.