Alllison Transmission Problem! Help wanted

I have an older Monaco with Cummins and Allison 4 speed, auto. just recently i found that when i shift down to 1st gear to go down a steep hill, there is no holdback! It will holdback in 2 nd gear but not first,
It will go up hill in first gear but when coming down it feels like it starts to freewheel.
Fluid has been changed, as was the filter but still does not hold in 1st gear. Any help appreciated.
Its a 8.3 cummins 275HP 1991 model.
Tha John :( :( :(
Re: Alllison Transmission Problem! Help wanted

The transmission in your vehicle is probably an MT643 or an MT647. Those do not apply the lockup clutch in the torque converter in first gear. That's what probably feels like the sensation of freewheeling to you.