Was working with my HST tractor today and all was going fine until I was stuck in reverse. Pedal was stuck in reverse and would not go into forward. Finally got the pedal to move by depressing clutch pedal and shifting into neutral. Pedal was extremely stiff. Got tractor to near level ground and found hydraulic fluid sight glass was white. Added about 1/2 gal of fluid bringing fluid level into the sight glass. Pedal was much less stiff but tractor had no torque regardless of gear and engine RPM. Crawled the tractor onto trailer in high gear at very slow speed. Less than 250 hours on the tractor. No indication of trouble until today. Where does all that fluid go in a "closed" system with no indication of a leak? Seems like the hydraulic system is locked up. Clogged filter? Any suggestions?