My JD X580 mower deck won't engage when I pull the yellow knob lever. Not sure if it's a coincidence or not but when it stopped working I looked underneath & saw that one of the deck leveling bolts (left side when sitting on the mower) had broken so I replaced that along w/ the concave washer & nut. Everything else seems to work fine so I'm thinking PTO switch or PTO clutch? Any way to test these to see which one it might be? Or any other ideas of what it may be?