The internal air passage valve is probably stopped up or a little slow. When the car has sat for a long time the fluid drains back out of the converter due to bad seals and design. When started and driven the transmission must refill the torque converter which now has some air in it. If the trip is very short, all the air is not purged and is compressed. If parked prior to the completion of the purge it blows the flud back into the pan and in some cases out the filler. This action is typical for all the very early PG's  
RAY Chevradioman http://www.vccacolumbiariverregion.org/1925 Superior K Roadster 1928 Convertible, Sport, Cabriolet 1933 Eagle, Coupe 1941 Master Deluxe 5-Passenger Coupe 1950 Styleline Deluxe 4-Door Sedan 1950 Styleline Deluxe Convertible 2002 Pontiac, Montana, Passenger Van 2014 Impala, 4-Door Sedan, White Diamond, LTZ 2017 Silverado, Double Cab, Z71, 4X4, White, Standard Bed, LTZ If you need a shoulder to cry on, pull off to the side of the road. Death is the number 1 killer in the world.
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