Classic/Antique Car Repair: powerglide automatic, converter drainback

Question
I have A 1964 chevy nova that after it has not ran for a month or so a large puddle of transmission fluid leaks out. it appears to be comming from the top vent. any suggestions?

Answer
This is a very common problem with most automatic transmissions when they get a lot of miles on them.  The symptom is called "converter drainback" and is a result of the fluid that is supposed to stay in the converter being allowed to seep back into the pan at the bottom of the transmission.  That pan is only designed to hold about 5 quarts, and so when the fluid in the converter drains back into the pan, there is no room for all the fluid, and it comes out of any orifice it can find - the air vent is a typical place.

There are two things you can do about this:  the easy thing is to just drive the car once a week or so, and never let it sit long enough for the pan to overflow.  The hard thing is to remove the transmission and have it rebuilt, making sure the mechanic knows it has this symptom, because the parts he will have to replace are more than what he would do in a normal transmission rebuild.

Dick