Need help on Case 550e trans

The dozer is a 95 year model. It was working fine and then it just acted like it did not want to go. Had to load it on a trailer to get to the Shop. I could hit the gas and then pull it to first gear and it will move about ten yards. I checked the filter under my feet cleaned and reinstalled it and it still will not go. I changed the transmission filter also. The filter under the seat only had about a quarter of it full with oil. Any help with this would be great. I did check fluid too. Thinking I need to check the pressure