riding mower?

I have a Craftsman 22 hp riding mower with automatic transmission that had a drive belt that was worn and would not turn the transmission. In taking off the drive belt, there was a lot of belt 'string' around the bottom of the transmission pully, which I removed. I replaced the drive belt and it ran fine, but after I found that the transmission was leaking. I had it checked at a service centre and they said it was leaking from the oil vent. They said the oil quantity was still fine and ran it for about a half hour without a leak. I then took it home and used it for about an hour and a half the leak returned. Any ideas?