I have a 1966 Chevy pickup with a powerglide in it. As far as I know it is original to the vehicle.

When you take off it runs great in low gear, then when it shifts in to high it will accelerate for a few seconds then begins to slip. It feels like you shifted into neutral.
Doe’s anyone know what causes this? Will it help to change the fluid and filter?
If it makes any difference I think the pickup sat in someone’s barn for a couple of years.

most likely time for a rebuild,high cluthes worn.those had a wave steel in high to soften shift,this caused slippage.if original im sure there are other worn parts

Over a long period, the piston sealing rings [which are rubber] become hardened and no longer seal. Plus, as Rusty said, clutches are worn.

Being it is original to the truck it sounds like it is time to show it some love with a rebuild and maybe some modern upgrades

If the shifter cable is even slightly out of adjustment, it will cause the shift flare, also.

Original trans??? 56 years old???. It owes you nothing. Needs a bench job... Problem being the older units with the paper filtering. Non screen
will varnish up and cause issues. If you decide to do a F/F. The contents in the bottom of the pan
will put you in the right direction.

This message has been edited. Last edited by: TD3550,