A-500 With Reverse Issues

Fluid won't make any difference in what is happening with reverse.
Most likely there is a problem with the rear servo sealing, and they do even crack.
Fortunately you can get to it to find out pretty easy when you have the pan off and the same part covers A500 42RE 42RH 44RE A727 A518 46RE 46RH A618 47RH 47RE 48RE A904 30RH transmission, so they are easy to find.

Most would replace it with an upgraded billet unit.