Originally Posted by MolaKule
I guess my question would be, if moisture ingress is the problem, where is the moisture coming from?
If these transmissions have a dipstick, then the dipstick seal must be allowing air to bypass it and suck in moisture when the fluid cools.
It probably has a breather like a differential has. My last truck, an 07 tacoma had a transmission breather that was tucked between the engine and firewall. I would pump fresh ATF in through the cooler lines without removing the fill plug and I could hear air escaping through the breather as I was pumping. Even "sealed" transmissions have to have a breather of some kind or they would leak around or blow out the seals as they warmed up and the fluid expands. As the fluid cools and contracts, it makes sense that air must enter the trans through the same breather to prevent the transmission from having vacuum (negative pressure).