To bleed air out Jack back tires up off ground, pull the release lever for transaxle (like you are trying to push by hand), move control to full forward hold for 5 seconds then go full reverse for 5 seconds, do this 5 times.
Push release back to driving position then repeat the same as above.
If you can't find release just do it without
You need to have tractor on Jack stands or something that is stable so it won't fall down when you have it in full forward or reverse.
If this doesn't work it might be an internal and you will need to take transaxle and pump out and rebuild it.
Did you check the drive shaft from engine to pump ?
I worked on a Cub Cadet a few years ago that had a pin sheared at the engine side, it would turn pump at idle with just a little movement on control, but when you went half way or full forward it would start slipping and not turn pump.