Chevrolet 2006 Corvette Owner's Manual page 314

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 about one-quarter turn and
then stop.
If you hear a hiss, wait for that to stop. A hiss
means there is still some pressure left.
2. Then keep turning the pressure cap slowly, and
remove it.
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3. Fill the coolant surge tank with the proper mixture
until the level inside stabilizes at the FULL COLD
mark on the front of the surge tank.