Chevrolet 2004 Corvette Owner's Manual page 275

1. Park the vehicle on a level surface. 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.
3. Then fill the coolant surge tank with the proper
mixture, to the FULL COLD mark on the coolant
surge tank.
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