I assume you mean a John deere. Have you checked the screen and filter? Then check flow and pressure on line up to main pump. If the tractor has been split there are a lot of steel lines inside that are hard to keep in place when putting back together. Does it have the hi/lo powershift?

 

Hi, yeah John Deere. I ended up sorting the problem after some googling and reading the service manual, my tractor has the bigger oil pump on it and basically it pumps more than the trans pump can keep up with. I plumed the return back into the filter canister so the oil goes back to the pump instead of back into the transmission housing. Works fine now, surprises me that the offered a large pump that effectively is useless with constant flow.