Have you done any work on the drive wheels over the winter?
I ask because it has happened a few times where someone removed a drive wheel to have a tire fixed, and the axle key fell out unnoticed. later when the wheel was put back on, without the axle key, the axle shaft would turn inside the wheel hub but the tractor would not move.
Have you checked the oil level in the trans?
Have you tried doing the purging procedure?
Usually it is only necessary to purge air after the trans has had the oil drained and refilled, but it may be possible that the pump has somehow "lost its prime" and become filled with air that needs to be purged.
If the tractor was parked over winter at something other than a level angle, plus having a low oil level, that scenario might have conspired to allow the pump to empty itself of oil and fill with air.

This post was edited by mownie on Wed, May 7, 14 at 1:14

Mownie: Yes -- I replaced a tire on one of the drive wheels. You're brilliant! The axle key did indeed fall out when I put the wheel back on.

It was a quick fix, thanks!!!

Let me correct your grammar just a bit.
It should be......"Mownie, you are..........experienced".
You're welcome.

My Craftsman 30" 13.5 hp hydrostatic mower is doing this very same thing. The drive wheel is ok, we replaced the drive belt too. Still barely moves. I thought it must be the transmission fluid but I cannot find any fill tube or drain. Can you help me out?

I have a Murray Hydrostatic 42” 14.5HP, new tranny oil, all linkages are connected, fan blades spin very fast and makes a whine noise but with the wheels off of the ground they barely spin full throttle. reverse does nothing. Is there a seal that’s bad inside the transmission? Any info would be greatly appreciated!