I had a 915 at one time. Sold the combine and kept the manual. If its the same as a 815 then the valve is on the left side behind the operators platform. Adjust the screw clockwise to slow the drop. Mine didn't have springs on the cylinders. I think that was if you had the automatic height control. As Ohio Farmer said, you had to block off the fast drop valve if you were going to run a cornhead. This is information that I found in my manual, which may or may not pertain to a 815.