The transmission in the Was Junk truck maybe junk again

[WZ JUNK]

I think the transmission in the old truck has transitioned  back to Junk from Was Junk.  It is a 78 model 350 turbo.  The last time I drove it, it started slipping and would not pull itself.  After I shut the engine off and gave it a few minuets, it would work again.  After a few cycles of this, I made it home.  Today I removed the pan, changed the filter, and put it all back together, and then I topped off the fluid.  The filter was really dirty, partial plugged with trash, and there was the usual trash in the pan. It did not look like metal shavings though.  I thought the problem was the clogged filter but when I test drove it, it started to slip again after about 10 minuets of driving.  I checked the fluid and it was showing a lot over full on the dipstick.  After it set for a few minuets, I started the engine and checked the fluid again and it was right on the full mark. I nursed it back home.  So something is going on, but I do not know what the problem is, and how to fix the transmission.  A friend built this one for me about 25 years ago and it has performed flawlessly all these years of abuse.  

'One other thing.  Recently I added a chemical to make the seals more pliable as it was leaking a little fluid after it set for long periods.

Do any of you have any thoughts?

[Crosley.In.AZ]

Do any of you have any thoughts?

Yes.  Pull the transmission out.  Open and inspect.

[enjenjo]

I think I still have a rebuild kit for it .

[WZ JUNK]

Do any of you have any thoughts?

Yes.  Pull the transmission out.  Open and inspect.

Thanks,  that will be the plan.  I talked to the son of the man who rebuilt it for me years ago yesterday.  I will pull it soon and take it to him.