Need help zero turn hydro fluid refill

[pa28pilot]

Changed the filters on both hydrostatic units today. Operators manual says to refill through the overflow tank but the fluid just sits in the tank. Seems to be an air lock tank doesn't drain. Is there a vent screw on the transmission that needs to be removed? I am really stuck. Help!!

 

[JTKub]

I assume this is in regards to Hydrogear EZT drives?

Per the service manual you need to take the fitting out of the top "fill port" to fill the units.

Joel