OK, so your 1963/64 Cadillac has a Park Brake. With the car running, it releases the brake when the car is running and you put it in a Drive or Reverse gear. Cool. That will help prevent driving the car around with the Park Brake engage. This also serves another function. What is it?
If the main brake system fails, the parking brake can be used to stop the car.
Bingo! It can be used as an auxiliary brake system. If the brake did not release in a drive gear it could not be effectively used as a brake as the rear wheels would simply lock up. This is one reason Cadillac advertised their brakes as a triple brake system when the new dual master cylinder systems came out in 1963. It is a very common sense function but based on some of the previous questions I thought some of you might over think it!
.... and by the way....
HAVE A CIGAR!!!!!