GMC 1998 Savana Van Owner's Manual page 220

Haw
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
ROW
push
down as you
turn
it.
Remove the
pressure
cap.
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