THM 350 issue with 85 C10

[peresich]

Alright, I got my first major issue to deal with on my new toy. Friday afternoon, as I was driving my daughter home from school, I lost 2nd and 3rd gear. 1st and R are still working. I tried to manually shift out of first but got nothing. It was cute watching my 10 year old start to panic as I drove down the highway at 20 MPH with the blinkers on.

"Daddy, what are you going to do?", she asked.

"Well honey, I am going to drive it in first gear back to work. Get your brother to come pick you up. Then drive it in first gear home tonight and take the lay on the floor in the garage and work on the transmission. If I didn't expect to spend some time getting dirty, then I wouldn't have bought a truck this old."

Anyway, I dropped the pan off of the transmission after draining most of the fluid out. The fluid was still bright red, so nothing seems to be burned up. Unfortunately, I got the wrong filter at the parts store and got sidelined a good chunk of the weekend with a stomach bug so the truck is still up on the ramps today. I am planning to put the new filter on and fill her back up with fluid this afternoon, but I figured I would ask if anyone had any advice on other things I should check out first.

I didn't see any other obvious signs of leakage besides the pretty rotten old gasket that was on there. It has to be the factory original because it looks ancient. There was a hard line running into the side of the transmission body that caught my eye as a potential weak spot, but I didn't see any fluid around the connection and it felt tight to me. Any tricks the old pros want to share with the rookie before he puts the pan back on for the first time?