I just put up an article on my website about replacing a fuel level sensor on my 1999 Buick Regal. The problem of the level being full matches my experience, too. If the sensor is similar to one I bought on ebay, it needs more weight on the float arm. Check out my article and scroll to the bottom to see what I did.
http://www.redhouseon7th.com/redhous...sor/index.html