An overfull condition will cause the fluid to foam and expand. The lubricating quality of the fluid will diminish because of the air that is in suspension in the fluid.
Trans. slippage will also cause the fluid to overheat and expand.
The expansion will force fluid out of the vent. A properly functioning
trans. shouldn't be forcing fluid from the vent. Some of the newer C4s don't have tubing for a vent, but just a small vent cap.
A dipstick that is for another model or incorrectly marked as well as a poor quality fluid that is lacking the antifoaming additives needed could be culprits.
I am fortunate that I had the experience to eliminate the the leak prone kickdown shaft leak on my own, but I want to say thanks, tripleblack, for your efforts in helping others with this chronic leakage problem. It's really nice to have a 40 plus year old car that doesn't leave its mark on the pavement. Especially when I park on one of my friend's driveways.