problem could be at the governor.  im thinking ag talk had a good thread on this in the last 6 months or so.  it might have been a youtube video I stumbled across, but i think the governor is supposed to stop sending air to the dryer but didnt because something was sticking.

i had an issue like yours with the dryer when a rubber lined air line between the governor and the dryer collapsed internally and would let air go one direction, but not the other direction to tell the governor to stop sending air.   simple check is to unscrew the air lines between the two and blow with an air gun both directions.

im not a mechanic.