I had the same problem with my 9n. When brush hogging it was all but impossible to get the brush hog to stay at a set height, it would move either up or down. I finally corrected the problem by adjusting the main spring which is located externally, above the differential housing, where the top link hooks up. To adjust the spring, remove the pin that holds it to the bracket that the top link mounts to, then turn in untill there is no end play, but you can turn the spring around with your fore-finger and thumb. It took care of the problem for me, I hope this is helpfull.