Recently changed hydraulic fluid, cleaned filter - everything worked fine. Used brush hog for about 1/2 hour, put the tractor away in barn. Now 3 point hitch won't lift brush hog. 3 point will go up slowly with brush hog disconnected, but won't go up if I stand on it. FEL works fine. Can anyone help? Does the hydraulic system need to have air bled after it is replaced?