It sounds to me like you have a leak somewhere in your brake system. I think you are losing brake fluid and that would explain the fact that it's getting worse. Locate your brake fluid reservoir and fill if needed. This will give you a temporary fix but if there is a leak it is only going to get worse and leak faster. Take the tractor a repair shop where they'll inject a special dye into your braking system and then any leaks will show up under an ultraviolet light. Hope this helps:)