drive belt in mower deck will pop out of pulley on the left side of the tractor and thereby disengage cutting blades. i've taken bracket off and put belt back in pulley and wiill work for 3/4 uses before the same thing happens again. noticed that the extension spring that connects to the pto cable is bent and somehow is responsible for a loss in tension to the pulley but i told that there could be other causes. like how did the extension spring get bent?