Good Morning; we were reading about transmission leaks over the last couple of weeks. I contacted Don Verity of the 300 Club International to help in the discussion. He is the 'go to' guy on this stuff. Having now read it, I have some work to do on mine. Happy tails.
On Torqueflite Drain Back.
What is actually draining back is the torque converter. What happens is that the converter holds about 2 thirds of the fluid in the transmission. That equals about 4 quarts above the center line of the converter. This is quite a lot of weight pushing down. All transmissions have a vent to allow for expansion and contraction of the fluid as it heat up and cools off. Any place that air or oil can leak through a seal, will allow the converter to slowly try and equalize it's level. There are numerous places on the trans where air can leak by, including the pump gears (to much clearance), drive sleeve seal, input shaft seals, regulator valve body, and stator support seal. The transmission case eventually gets over filled, and fluid try's to find it's way out wherever it can. This usually ends up being the kick down shaft seal, or the shift cable. The shaft seal is not very big, and shaft is constantly moving back and forth with throttle position. To compound things, the shaft goes through a bore in the valve body that usually gets loose with age. This causes the shaft to egg out the seal over time. The best way to keep the fluid where it belongs and to make the seal last longer, is to first make sure the steel washer is on top of the seal under the kick down lever. This washer helps to keep dust and dirt out of the seal. Also make sure there is minimum up and down play in the shaft when you tighten the clamp bolt. Pulling up lightly on the shaft while tightening the bolt will keep play to a minimum and help keep the shaft stable, just make sure it does not bind through it's range of travel. The shift cable O-ring can leak, and also the casing itself, if it is damaged. The oil pan is usually not a problem if the gasket surface is straight, and a quality gasket it used. I never use any sealer on the gasket, and torque to 12-13 foot pounds. With a cork gasket I use less, usually around 10-11. The main way to combat drain back of coarse, is to drive the car regularly.