Don Verity writes:
On Torqueflite Drain Back. What is actually draining back is the torque converter. The converter holds about 2/3 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 to equalize its level. There are numerous places on the trans where air can leak by, including the pump gears (too much clearance), drive sleeve seal, input shaft seals, regulator valve body, and stator support seal. The transmission case eventually gets over filled, and fluid tries to find its 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 the 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 its 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 course, is to drive the car regularly.