Chevrolet 2003 Astro Owner's Manual page 254

How to Add Coolant to the Radiator
You can remove the
radiator pressure cap
when the cooling system,
including the radiator
pressure cap and
upper radiator hose,
is no longer hot.
1. Turn the pressure cap slowly counterclockwise until
it first stops. (Don't press down while turning the
pressure cap.)
2. Then keep turning the
pressure cap, but now
push down as you
turn it. Remove
the pressure cap.
If you hear a hiss, wait for that to stop. A hiss
means there is still some pressure left.
5-37