675b

I have a 675b I brought and having a issue with it bucket all works as it should but when you go to move the handle to drive the tires the left side works as it should but not the right side so I swap drive motors from each other and still the same issue right side starts to move but it slips like theirs not enough hydraulic fluid their if you hold the handle so far back and keep it their it will rotate as it should but that’s it so i took the front pump off and spilt the trans in half had it look at and guy said it was fine didn’t see anything wrong so but it back together and still doing the same thing