When downshifting on a hill trans doensn't hold car back acts like neutral

[1997GSX]

Hey guys, my dad just rebuilt the trans in his gtp because it was not shifting in to overdrive.

It shifts fine through the gears but if you manually select 2nd when going down a large hill to hold the car at speed. The trans acts like it is in neutral. When he gets back on the throttle it still acts like it is in neutral.


He bought the car blown up with known trans problems.

[1997GSX]

bump