Changed trans fluid in my TH400 and now it wants to slip and not get into gear.

[HeavyHauler]

I recently changed the trans fluid in the TH400 of my 1968 C10 shortbed fleetside and now it wants to slip and not get into gear. I feared this was going to happen. I decided to fix a trans pan gasket leak and changed the fluid in the process. The trans has alot of miles on it and most likely needs rebuilt but I was hoping to get some more cruising in before swapping my factory 396/TH400 for a 454/4L80E combo. So that I could rebuild my original drivetrain and set it aside if I wanted to return it to factory specs in the future.


Any suggestions about my TH400.