Chevrolet 1997 Corvette Owner's Manual page 231

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 about
one-quarter turn to the left and then stop.
3. Then fill the coolant surge tank with the proper mix,
up to the FULL COLD mark.
2.
If you hear a hiss, wait for that to stop. A hiss means
there is still some pressure left.
Then keep turning the pressure cap slowly, and
remove it.
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