Are you talking about bleeding the hydraulics? Not sure what year you have, but the manuals are generally available for download on the Scag site.
Related to the hydraulics, the manual for mine says:
4. Fill the reservoir to 3-1/4" inches from the top of the filler neck with 20W50 motor oil. 5. Replace the reservoir fill cap. Start the engine and drive forward and backward for two minutes. Check the oil level in the reservoir. If necessary, add oil to the reservoir.