Chevrolet 1998 Prizm Owner's Manual page 210

How to Add Coolant to the Radiator
1. You can remove the radiator pressure cap when the
cooling system, including the radiator pressure
cap and upper radiator hose, is no longer hot.
Turn the pressure cap slowly counterclockwise
until it first stops. (Don't 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 pressure cap, but now push
down as you turn it. Remove the pressure cap.
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