i'm going to back valley on what he said about not workin at all after fluid and filter change it could go all together.if you really want to change fluid just drain fluid and get some carb cleaner and spray the filter clean as you can still on the valve body without unbolting it.had a c4 once in a car that worked pulled motor and trans out and put it in another car 2weeks later and trans made it out the driveway then quit i'd guess from tilting and rattling it around broke some of the trash around.i'd try the brake fluid trick first it wont hurt or cost much like i said if its a seal it will help unless trans is to bad allready
good luck