I would check the radiator overflow bottle. If it is full then I would suspect the radiator is clogged or has a worn radiator cap, or that there is not a airtight seal between the two. You have a Yanmar engine in your tractor and I believe that you would need a 13 psi overflow type cap. If the water is not pulled back into the radiator then you loose it the next time it builds over 13 lbs of water pressure. You can try to drain and flush the system or just pull the radiator and have it rodded out at a shop. These are the simple things that can cause the problems you are having and they are cheap to address before you move on to the more serious posibilities.