Classic/Antique Car Repair: Turbo 400, chevelle ss 396, 1970 chevelle ss 396

Question
I have a 1971 W-30 4-4-2 with automatic transmission.  When it sets for a period of time, usually over 3 or 4 months, the transmission dumps fluid.  I had a 1970 Chevelle SS 396 with the same transmision that did the same thing.  Is this normal and is there anything I can do to prevent it?

Answer
Hi Billy:
The seals in a TH-400 work best when heat is applied to them. The expectation of GM was these vehicles would be driven and the heat from the transmission would keep the seal swollen and leakless. Having them set for that length of time weaken the transmission seals as well as the head gaskets, although the head gaskets usually takes a little longer to leak. RECOMMENDATION...Drive her weekly if possible.