Why wont my transaxle engage after fluid change?

I have a hydrostatic lawn mower Murray model 46801x6b and it has transaxle made by Spicer model 8158338 i think. or it could be 1ZY2F174YN91012096.

Anyways I removed it and changed the oil in it and replaced its fan after digging it free of 15 years worth of dirt and grass. I must have to bleed the system but am not sure how to do it.

Any suggestions would be appreciated.