Chevrolet Corvette Owner's Manual page 356

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.
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2. Then keep turning the pressure cap slowly,
and remove it.
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.
I n f o r ma t i o n P r o v i d e d b y :