Is this trans basically stock or is it modified any ? The front clutch is on in both drive and reverse so a problem in the front clutch could be cause. Reverse also needs the L/R band but you dont need the band for first in a stock trans as the overrunning clutch (sprag) will hold in first. So it would still have first in even if the L/R band had a problem but no reverse. Just off the top of my head ts seems it has a problem of the front clutch not working. I cant think of hard braking causing a problem in the front clutch ? Ron

I realize I forgot this is an overdrive trans as it could also have a problem in the overdrive unit but I cant think of what would cause both reverse and first to be lost. If the direct clutches had a problem it can cause a reverse problem but the overrunning clutch in the overdrive would hold for first to work. Did you replace the complete overdrive unit or repair it ? Ron