Guilty but no longer. To be honest I thought that thing was like some kinda button knob to engage some mystic feature if the tractor I never used...So, you have never checked the hydraulic oil level?
Have you checked the oil level in the front axle?
Twist the yellow dipstick and pull on it to remove it. While it is out, put some hydraulic oil on the o-rings before reinserting it. The front axle dipstick is removed the same way.