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Just got done with a frame-off on a 79 F250 4wd. When driving it, once it's warm, as I'm slowing to a stop, it will slip out of drive and usually re-engage with a loud clunk (driveline/diff noise). It also happens when I'm stopped. If I manually put it into 1 or 2, it's fine. This lead me to believe it may be a vacuum issue, since it only affect drive.
I have a pressure tester on the way to test the pump and a black stripe vacuum modulator to replace the current, the old modulator wasn't changed when the transmission was rebuilt. Am I going in the correct direction or should I be looking somewhere else?
Admittedly, I did 99% of the work on this truck myself, including rebuilding the transmission. This was my first auto tranny rebuild. I also rebuilt it 3 years ago, but I remember air testing it back then. I used the coveted C6 rebuild book and I'm an experienced mechanic, although not immune to mistakes. lol
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