GMC 1996 Sonoma Owner's Manual page 204

How to Add Coolant to the Ra
I . You can remove the radiator pressure cap when the
cooling system. including the radiator pressure cap and
upper radiator hose, is
1 1 0
longer hot. Turn the pressure
cap slowly counterclockwise until
it
first stops. (Don't
press down while tu1-nin2 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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