Buddy of mine has an old 825 bobcat that quit moving on one side. It was running fine when he parked it and the next day the right side wouldn't pull. You can hear it kind of kick in and out and try but not move. The hydros have been rebuilt in the last few years. It's actually a pretty decent machine till this happened.
Could have a broken chain or something with the linkage. It's a simple system, so unlikely that the hydraulic portion would fail suddenly.
I wonder if the front pump is connected to the side that won't drive. Could be the coupler between the pumps stripped. Do the hydraulics work?
I will ask him. It kind of wants to pull then turns loose.
My guess would be that something is slipping on splines, if it tries, then pops off...
Hydraulic system works, it seems to be in the hydrostat drive. When you first start it you can almost get movement, it will try then turn loose. After a minute or two it does nothing on that side. I've had a spline strip on drives before and it doesn't act like that. A valve or relief valve is my thoughts but I know nothing about how the 825 is made.