When faced with a rusted nut or a pulley that is rusted on a shaft, one thing that has worked well for me is to mildly heat them with a propane torch and melt some wax (hold a candle to the hot metal) onto the shaft. The wax wicks down the shaft and provides enough lubrication to get the stuck part off.
Maybe this technique would work for the riser shaft.
All my best, Dean
