okay I will tell you how I did mine but it may be a bit different than yours
1. jack the back so the drive wheels are off the ground jack the rear of mower off the floor to get the weight off the tires.
2. Open the hydro fluid cap so air can escape
3. Crank the engine and run it at idle, and slowly move the sticks forward and reverse to circulate the oil through the system and work the air out. do it like that so as not to put extra strain on the motors and pumps trying to move with the weight of the mower and the operator on the system
4. do this for about 10 min and the should be fine if they are not you may have to do it again
Be sure to keep the hydro tank topped-up or you will pump more air into the system
that is how I did mine a couple weeks ago and it is running fine
Like I said I am not familiar with these types of ZT's so it might be different on yours.....
try this and see if it does any thing.