The HST fluid & filter should have been changed at the 300hr maintenance, was this done and did you use SUDT?
If so, it's not scheduled for another change until 600 hrs but that doesn't mean there isn't a problem w/the filter, sump screen or even the fluid.
One test you can try to make sure the brakes aren't sticking inside the HST is to jack the rear up off the ground and see if you can spin the wheels easily. (HST in neutral ofcourse). If they spin easily it's then safe to say that the integral brakes aren't sticking.
In the few cases where I've seen posts on HST failures, the failure was always preceeded by unusual sounds. Either a loud scrapeing metal on metal type sound or loud squealing sounds. Again HST failures are pretty rare and they just didn't happen all of a sudden.
Good luck