My 1972 Road Runner (Torqueflite A-727 Automatic) transmission slips (high RPM without any power through the trans) but only in high (third) gear.
This happened after I noticed a trans leak while driving (after storing it for the winter).
I added more auto trans fluid and there is no more noticeable leaks.
It seems not to slip in first or second gear and up shifts/down shifts fine.
Would a complete fluid replacement help/cure this or do I need a rebuild?
Is it possible I cooked third gear only by running low on trans fluid.

Help ... this is my first Mopar automatic trans and I'm not sure what to do.
Thanks!