Iseki sf310 mower

[paulcrox]

Hi all, had a sf310 sitting in shed for a couple of years, needed a new a new deck. Finally one came up at a good price.
installed it, took it out, after 30 seconds, lost forward reverse in it.
was wondering if issue could be air in the hydraulic lines? Or if anyone else has an idea. Hope it’s not HST transmission gone.
does anyone have the correct procedure for purging air?
cheers
paul