Cadillac 2005 DeVille Owner's Manual page 302

1. You can remove the coolant surge tank pressure
cap when the cooling system, including the coolant
surge tank pressure cap and upper radiator
hose, is no longer hot. Turn the pressure cap slowly
counterclockwise (left) until it first stops. Do not
press down while turning the pressure cap.
If you hear a hiss, wait for that to stop. A hiss
means there is still some pressure left.
2. Then keep turning the cap, but now push down as
you turn it. Remove the pressure cap.
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3. Fill the coolant surge tank with the proper mixture
to the FULL COLD mark on the side of the coolant
surge tank.