on my 65 Skylark with the ST300 trans twice now has started slipping. Both times I was down over 2 quarts. No obvious leaks. Front and rear seals are both dry. Doesn’t look like the cooler in the radiator is leaking either. Any guesses?
The vacuum modulator diaphragm can have a pinhole leak and slowly pull ATF into the intake where it is burned in the cylinders. Remove the modulator and point the vacuum nipple down. If any ATF drips out, you've found the issue.