My transmission leaks when it sits for a while

Question:

The C4 automatic transmission on my 1965 Mustang will leak over a quart of fluid from the breather tube after sitting for three or four days. I do not over-fill the transmission and other than that, it shifts properly and goes into drive or reverse properly. One mechanic said that oil is leaking back from the torque converter through a check valve after the pressure on the valve becomes less from sitting for a few days. Another mechanic said it would not go into drive properly if that valve was the problem, and he thought it may be the shaft that has some dirt or corrosion. The car did sit for eight years before I recently bought it. Any help on this would be greatly appreciated.

Answer:

I believe the problem is the check valve that’s supposed to prevent the fluid in the torque converter from leaking out when the engine is off. If the valve has only a very minor leak then the transmission pump would easily build up enough pressure when the engine is running and the transmission would go into drive and function normally. But that minor leak would still let a noticeable amount of fluid slip by when the engine is idle for a few days or longer. The fluid that comes out of a torque converter inevitably runs down into the pan and when the level in the pan rises sufficiently some fluid will come out of the vent tube.