Fill it with Dex 3 until it reaches the full mark on the fill spout. Shouldn't take that much if you are just changing a hose. Less than a quart. You might have to start the engine and turn the wheel lock to lock a few times to cycle out all the air.
In my Haynes manual it says NOT to put it to full lock when you a getting the air out. Its probably a quart low right now anyway. So I picked up two qts. Ill just add it till its full.