If you bought the tractor used it may have had automotive brake fluid added to the master cylinder. Automotive brake fluid will dissolve the seals in the brake cylinders located in the rear end and it will leak fluid there. Don't remember from when I was told if you will need to pull the axel or lift the rear end cover to repair the seals. As usual large labor expense and small parts expense.I repeat DO NOT USE AUTOMOTIVE BRAKE FLUID!!!! Not on the hydraulic clutches either.