72 Dart 904. I was doing regular maintenance today checking fluids. I was out running around several places. I have a small leak and have to add AT fluid once in a while. The trans was hot and cycled through gears and was at the add 1 pint mark. I went to the parts store and put in a pint while there. Went to the grocery next door down. When I started back home the trans shifted to third and nothing. Stopped and started again trans shifted 1,2,3 and nothing. The engine just reved like in neutral. 1,2,R all work. I tried manually shifting but still nothing. What might cause no third? Adjustment or hard part failure?