Problem: at times, the hydraulic pumps will loose function. They wont allow the motors to go forward or backward. The motors free-wheel. This situation first started when I changed the two hoses on the left pump. I ran the mower with the wheels suspended for 5 minutes and worked the levers. I drove it around a bit, up into the truck and all seemed to be well. I had a prior leak so I added hydraulic fluid. I didnt know that this system used Mobil-1 15w-50. The next morning I tried to pull this mower out of the truck to change blades, but there was no function on the hydros. I could not move the mower forward or backward there was nothing. I pushed the mower out, called the manufacturer and was told to change the fluid out with pure Mobil-1. I did that, taking the hoses loose and draining them along with the tank. I also took one of the wheel motors apart to see what was inside (curious). I put everything back, filled up with Mobil-1, purged the air, test drove and put on the truck. The next day I went to the first job and tried to move the mower when the same problem surfaced. The hydros didnt want to respond. I played with the levers a bit and finally it creeped out of the wheel lock and I headed down the ramp. As I went down the ramp, I lost all hydro control and the mower free-wheeled straight down. I was parked in a driveway on a hill and was heading right for the road. Fortunately I was able to get the hydros to work right before I was going to abandon the ship. I am really puzzled as to why this is happening. The hydros work strong, just like usual, but the potential for nothing seems to be ever present. These are 10cc Hydro Gear pumps. Any suggestions would be welcomed.