GMC 1996 Sierra Owner's Manual page 233

How to Add Coolant to the Radiator
I .
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.
j
If y o u hear a hiss, wait for that
t o
stop.
A
hiss means
there is still some pressure left.
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