GMC 1996 Suburban Owner's Manual page 232

How to Add Coolant to the R
.tor
2. Then keep turning the pressure cap, but now push
down as you turn
it.
Remove the pressure cap.
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.
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